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FS O PR O E PA G EC TE D 1 C O R R Inside the atom N All matter is made of atoms. Everything we see around us is made of atoms. When we feel U something, we are touching atoms. When we smell something, atoms have entered our nose and made contact with particular cells that absorb some of the atoms and then send messages to our brain. We ourselves are made from atoms. Atoms are continuously recycled. When we take a breath of air we are breathing in at least some of the same atoms that were breathed in and out by people who lived on our planet thousands of years ago. So how does it make you feel to be breathing in some oxygen atoms that were once breathed in and out by Beethoven or Einstein, or by a dinosaur? 01_CRA_IS9_77563_TXT_LAY.indd 2 28/08/13 2:54 PM Development of the atomic model 1.1 PR O O FS Describing the structure and properties of atoms is extremely difficult because we do not have the technology to see them, and yet atoms were first described thousands of years ago. Our understanding of what atoms are and their structure are described using theories and models. These models have changed considerably over time as scientists have discovered more evidence. Students: PA G E »» identify that all matter is made up of atoms »» outline the developments to the atomic theories and models as a process of refinement and review of the scientific community EC TE D Subatomic particles 1.2 Students: »» describe the structure of atoms in terms of protons, neutrons and electrons »» use models to describe the arrangement of subatomic particles in common elements (additional) U N C O R R As our understanding of the structure of atoms improved, the existence of subatomic particles was discovered. Subatomic particles, their numbers, proportions and arrangements define the atom in terms of the element, its mass, the overall charge and its properties. Radioactivity 1.3 Not all atoms are stable. Unstable atoms will decay to form different, but more stable atoms. This decay results in the emission of energy and particles. These emissions are called radiation. Students: »» identify that radioactivity arises from the decay of nuclei in the form of particles and energy »» evaluate the benefits and limitations of the medical and industrial use of nuclear energy 01_CRA_IS9_77563_TXT_LAY.indd 3 3 28/08/13 2:54 PM Experimental evidence is a key driver of science. As Richard Feynman (American Nobel Prize winner) once said, ‘If it disagrees with experiment it must be wrong’. But is there evidence that atoms actually exist? They are too small for us to see in detail, even with modern technology. So just imagine the problems that earlier scientists encountered trying to piece together ideas that explained the many and varied properties of substances. The discovery of atoms has been a journey involving many different scientists working in different ways, all gathering evidence and seeking answers to these questions – not just about whether atoms exist, but what atoms actually are. PR O O FS 1.1 Development of the atomic model PA G E The Atomic Theory of Matter O R R EC TE D The idea of the atom has been around for thousands of years. But the explanations of exactly what atoms are, what they themselves are made of and how they behave have changed over time as new scientific discoveries have been made. The technology that enables us to actually see individual atoms is only just being developed, but we still cannot see the particles within them, and so we use models to describe their structure and behaviour. It is these models that have been modified, and contribute to the Atomic Theory of Matter. U N C Democritus and ‘atomos’ Democritus, an Ancient Greek philosopher, was the first person recorded to have considered the presence of atoms. He suggested that if you took any substance and continued to cut it in half, you would eventually get to a particle that was too small to be divided any further. He called these indivisible particles atomos – atoms. He also suggested that all matter, even invisible gases, is made up of atoms. Dalton’s atomic theory In the early 19th century, over a thousand years later, English chemist John Dalton built on Democritus’s idea of indivisible particles. He also suggested that different substances were made up of different particles that had specific masses and properties – elements. In other words, the particles that made up gold were different to the particles that made up water. He used the term ‘atom’ to describe these tiny particles. Dalton also suggested that these different atoms could combine in regular ratios to make new substances – compounds. Dalton did not have overwhelming evidence for his hypothesis at this time. Not all scientists agreed with him and a long time passed before his ideas became recognised as an important theory. Imagine for a moment that you had never heard of atoms, chemical symbols or formulas. Nobody had ever called oxygen O2, water H2O or carbon dioxide CO2. This was the situation in the world of the chemical sciences in the early 19th century. 4 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 4 28/08/13 2:54 PM FS It was also known that another compound containing carbon and oxygen existed. This compound always contained 1.33 times as much oxygen as carbon when the masses of the elements were compared. This compound is now known as carbon monoxide (CO). Evidence such as this led Dalton to propose the law of simple multiple proportions. It means that when elements combine, they combine in simple ratios, like 2:1 as in water (H2O), 1:4 as in methane (CH4) or 2:3 as in aluminium oxide (Al2O3). This might seem obvious to us now, but we only know this due to Dalton’s pioneering atomic theory. Dalton’s theory gave scientists a way to explain the evidence about atoms. Figure 1.1 Antoine Lavoisier. O PR O Antoine Lavoisier, a French chemist, made accurate measurements of the composition of chemical compounds in the 1780s, about 30 years before Dalton’s theory. From this work, it was found that compounds containing more than one element always had the same relative amounts of each element. For example, the compound now known as carbon dioxide used to be called ‘fixed air’. This was because it was heavier than air and did not allow other substances to burn in it. It was discovered that the mass of oxygen in fixed air was always 2.66 times the mass of carbon in the compound. This is an example of what is now called the law of constant composition. E Activity 1.1.1: The case for atoms PA G Imagine you are acting as a lawyer in a courtroom trial and your task is to convince the jury that atoms are real. You have the following pieces of evidence: • Element can join together to form compounds. O O Figure 1.2 An oxygen molecule (right) is formed when one oxygen atom (left) joins with another. O H H • Water always seems to contain twice as much hydrogen and oxygen. EC TE D • When chemicals react with each other, the mass of the chemicals overall does not change. • Pure oxygen has the same properties wherever it is found on the Earth, and even in space. • Gases (some of which are invisible) have mass, and different gases have different masses. O R R • Under the microscope, tiny particles of pollen in water move in strange ways as if bumping into invisible objects. 1 Do you think any of these pieces of evidence would be useful in persuading the jury? If so, why? Figure 1.3 A water molecule is made up of one oxygen atom and two hydrogen atoms, always in the ratio 2:1. O Figure 1.4 A carbon dioxide molecule is made up of one carbon atom and two oxygen atoms, always in the ratio 1:2. C N U A scientific theory is written to explain existing evidence and observations. A good theory supported by a range of evidence can be used to make testable predictions. Ever since Dalton first proposed his atomic theory it was used to make predictions, and evidence that was not even available in Dalton’s time still supports the theory. However, theories can be amended as new and more accurate information is discovered, just like models can be amended. C O 2 Choose three of the above points and explain why you think they could be used as evidence to support the existence of atoms. Scientific theory O O O C Figure 1.5 Carbon monoxide is a compound made up of one carbon atom and one oxygen atom, always in the ratio 1:1. H H H C H Figure 1.6 Methane is a compound made up of one carbon atom and four hydrogen atoms, always in the ratio 1:4. 1.1 Development of the atomic model 5 01_CRA_IS9_77563_TXT_LAY.indd 5 28/08/13 2:54 PM O FS filed past his coffin prior to his burial. Dalton’s last act was to allow his eyes to be used for scientific research after his death. Ironically, his theory about the cause of his own colour blindness was found to be incorrect. DNA tests carried out on his preserved eyes 150 years after his death showed that his colour blindness was caused by a genetic disorder. Summary of Dalton’s atomic theory of matter • Elements are made up of atoms, which are extremely small particles. PR O John Dalton was born in 1766 in Cumberland, England. His older brother Jonathon ran a Quaker school in Kendall, near the English Lake District. Because Dalton was part of the Quaker tradition (Quakers did not follow the teachings of the ‘establishment’ Church of England), he was not able to attend or teach at an English university. Instead, when he was 27, he taught mathematics and natural philosophy at a college in Manchester. His earlier scientific work was not particularly successful. His main scientific focus was meteorology, and he also produced work on colour blindness, inspired no doubt by the fact that he was colour-blind. Dalton’s first chemistry-related work, when he was 35, was the study of the behaviour of gases in the atmosphere. This work led him to consider the idea of atoms, and it is this theory for which he is now most remembered. The work of many other scientists has been built on John Dalton’s atomic theory. When he died at the age of 77, after a series of strokes, he was extremely famous and 40 000 people • Atoms of each element are different from those of other elements, including their masses. • Atoms of a given element are identical to each other. • Chemical compounds are formed when atoms of one element combine with atoms of other elements in the same fixed proportions. EC TE D PA G Figure 1.7 John Dalton John Dalton E Deeper u n d e r s ta n d i n g • Atoms cannot be created or destroyed in a chemical reaction. Questions 1.1.1: The atomic theory of matter Remember R 1 Identify the philosopher who first described the concept of atoms. O R 2 Suggest a reason why Dalton’s ideas about atoms were not initially accepted. 3 Define the law of simple multiple proportions. U N C 4 List the formulas of: a carbon dioxide b carbon monoxide c methane Apply 5 The formula of water is H2O. Describe what the ‘2’ in the formula tells you. 6 Explain the difference between an element and a compound, and give an example of each. 7 Explain the difference between an atom and a molecule, and give an example of each. 6 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 6 28/08/13 2:55 PM Discovering more about atoms FS O PR O + _ Figure 1.8 Thomson’s Through even more experiments, cathode ray tubes. The Thomson also showed that the atom electron gun is on the left contained positively charged material, hand side of the diagram, and the parallel plates although it was not yet clear what this produce the electric field. material was. From this evidence, and As the electric field is turned on, the electron knowing that oppositely charged objects stream (blue line) is attract each other and move towards each deflected upwards. other, Thomson suggested that the atom was like a plum pudding, in which the positively charged material is the ‘pudding’ and the electrons are the fruit. This was called the Thomson plum pudding – model of the atom. – In 1911, Thomson’s former – student, Ernest Rutherford, – – performed an experiment to – – test the plum pudding theory. The results caused Rutherford – – to conclude that the atom is – actually mostly empty space. These observations led to the model of the Figure 1.9 Thomson’s atom known as the Rutherford nuclear plum pudding model of model. You will find out more about this the atom. model in the next section. U N C O R R EC TE D In the early 20th century, a century after Dalton proposed his theory, physicist Joseph John Thomson (known as ‘JJ’ by his colleagues) discovered that atoms were actually divisible and made up of even smaller particles. Thomson experimented with cathode ray tubes – a glass tube with electrical wires in either end and the air within the tube removed to form a vacuum. Passing an electric current through the tube created a fluorescent glow. It was a very old-fashioned version of a florescent tube light globe. Cathode rays containing electrons could be fired out of the end of the tube, so it also became known as an electron gun. These cathode rays could be focused to form a glowing dot on fluorescent material. However, the electrons could not bend around or pass through solid material and so Thomson concluded that electrons must be tiny particles. Thomson fired the electron gun near an electromagnetic field, and discovered that the rays of electrons were bent away from the magnetic field. From this information, Thomson concluded that electrons must be negatively charged. _ + E The Thomson plum pudding model Through a series of further experiments, Thomson discovered the negatively charged particles, electrons, were too small to calculate their individual mass. But, based on how much the rays bent in electromagnetic fields of varying strengths, he deduced that electrons must be about a thousand times smaller than a hydrogen ion, and must be coming from the atoms within the cathode ray tube itself. PA G Dalton only had access to a small amount of evidence compared to the information we have about atoms today. As scientific methods improved and technology advanced, more and more was discovered about the atom. This work further developed our understanding of the atom, and helped provide more evidence for the existence of atoms as proposed by Dalton. 1.1 Development of the atomic model 7 01_CRA_IS9_77563_TXT_LAY.indd 7 28/08/13 2:55 PM Activity 1.1.2: Brownian motion using diluted milk Predict what you might expect to observe during an experiment to observe Brownian motion. Use your prediction to formulate a hypothesis to test for the experiment you are about to conduct. Read the full method and consider any possible risks, and determine how you can minimise or eliminate them. What you need: microscope with objective of at least 20 × and eyepiece 10 ×, microscope slides and cover slips, full-cream milk, needle or fine wire, distilled water, petroleum jelly FS 1 Place a small drop of distilled water in the centre of a microscope slide. (Note: This drop must be very small so that no water escapes when the cover slip is added.) PR O 3 Using the needle, carefully stir the milk into the water drop. O 2 Dip the needle in the milk, and then quickly dip the tip of the needle with the milk into the drop of water. 4 Using the needle again, carefully line the cover slip edges with the petroleum jelly. 5 Gently lower the cover slip onto the drop of water containing the milk. 6 Place the microscope slide under the microscope and bring it into focus. You should be able to see the tiny oil droplets in the milk. PA G E 7 Wait for the sideways movement of the oil droplets to stop and look for the ‘jiggling’ motion of the droplets. If you observe this, you are seeing the direct action of water molecules on these oil droplets. • Did your observations support your hypothesis? EC TE D • Identify some ways of improving your experimental technique that might make your observations of Brownian motion easier. • What do you think causes this random motion? Questions 1.1.2: Discovering more about atoms Remember R 1 Outline why Dalton’s atomic theory was so highly regarded. O R 2 Describe Thomson’s plum pudding model of the atom. U N C 3 Describe where else you have experienced opposite charges attracting. Apply 4 John Dalton said the atom is the smallest particle that an element is made from. He said atoms could not be divided into smaller parts. This theory fitted the evidence available at the time but new evidence came to hand after this. Using your previous knowledge, outline what we now know about the atom. 8 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 8 28/08/13 2:55 PM 1.1 Development of the atomic model Critical and creative thinking 1 Identify the ratio of carbon to hydrogen in the compound methane. [1 mark] 11A Year 5 primary school class were learning about solids, liquids and gases. Their teacher told them that everything around them was made of particles they could not see. One student responded that this was silly, because if you can’t see it, it can’t be there. Write a paragraph to the student to persuade her that the teacher was correct. [2 marks] 5 Describe why it would be impossible to have a chemical formula with a fraction in it. [2 marks] Apply 6 Explain what causes the motion of tiny droplets of oil in diluted milk. [2 marks] O Making connections 12Use your understanding of atoms and elements to suggest reasons for the following: a carbon dioxide is a heavier gas than oxygen [1 mark] b hydrogen and oxygen can be produced from water [1 mark] EC TE D 7 Outline why the molecules of water are impossible to see, even with powerful microscopes. [2 marks] PR O 4 Recall what the ‘2’ in the formula CO2 represents. [1 mark] E 3 Recall the formula for the compound carbon monoxide. [1 mark] PA G 2 Identify which chemical law describes how the composition of compounds is always the same. [2 marks] Checkpoint FS Remember and understand Analyse and evaluate 8 Explain the difference between a theory and an observation. [2 marks] d at room temperature, hydrogen is a gas but water is a liquid [2 marks] C O R R 9 Describe why it was important for chemists such as Lavoisier to be able to take accurate measurements of the mass of elements within chemical compounds. [2 marks] c when methane is burned in oxygen, the gases carbon dioxide and water vapour are produced [2 marks] U N 10Describe the differences between a scientist and a natural philosopher. [2 marks] TOTAL MARKS [ /25] f the atomic model 9 01_CRA_IS9_77563_TXT_LAY.indd 9 28/08/13 2:55 PM As technology develops, scientists can explore deeper and deeper into areas once unknown. By experimenting on smaller scales with increased accuracy, it has been possible to discover information about the inside of atoms – the subatomic world. Here we encounter protons, neutrons and electrons, and realise that atoms are tiny systems made up of these subatomic particles, all interacting with each other. These interactions are predictable, based on knowledge about the mass and electrical charge of protons, neutrons and electrons. The properties of atoms are entirely dependent on subatomic particles, including their number and how they are arranged. Although we cannot observe the atoms themselves, the properties of these atoms determine the behaviour of the elements that we encounter every day. PR O O FS 1.2 Subatomic particles deflected (made to change course) by the gold atoms in the thin sheet of gold foil. Two aspects of the results surprised the scientists. The first evidence was that most of the alpha particles passed straight through the gold foil with hardly any deflection at all. Somehow they seemed to have passed through the gold, and therefore through the atoms that made up the gold, without ‘touching’ anything. More amazing was the second piece of evidence: some alpha particles travelling with a high amount of energy bounced straight back in the direction they had come from. From this evidence, Rutherford concluded that the gold atoms must contain a lot of space, but some areas of the atom contained a relatively large amount of positive charge to repel the positively charged alpha particles so strongly. Figures 1.11 and 1.12 show how the evidence from the gold foil experiment helped people change from using the plum pudding model of the atom to using an alternative model. This new model had the positive nucleus of the atom surrounded by a relatively large area of space containing the negatively charged electrons. It was a large shift in thinking about the structure of atoms. R Figure 1.10 Ernest Rutherford in one of his laboratories. EC TE D PA G Models are used in science to help understand, explain and predict. Modelling is an essential part of science. They can be used in a huge range of situations such as modelling the structure of the Universe, modelling the flow of energy within our own bodies and modelling systems too small for the eye to see, as is the case for atoms. What scientific models have you seen, or used yourself, to help explain things that happen around us? E Looking inside the atom O R Rutherford’s experiments on atoms Ernest Rutherford was born in New Zealand in 1871. His experiments changed the way people thought about the inside of the atom. At the age of 37, he supervised Hans Geiger and Ernest Marsden, who carried out what is known as the ‘gold foil’ experiment. They set up a very thin layer of gold and fired a stream of alpha particles at it. An alpha particle is made up of 2 protons and 2 neutrons, and has a positive charge. Detectors to record the alpha particles were set up around the gold foil to identify whether the alpha particles had gone straight through the foil or had been U N C Figure 1.11 If the ‘plum pudding’ model of the atom is correct, it would be expected that most of the high-energy alpha particles would move through the gold with only minimal or no deflection. 10 Figure 1.12 The gold foil experiment showed that high-energy alpha particles were deflected. 10 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 10 28/08/13 2:55 PM Activity 1.2.1: Modelling Rutherford’s experiment Rutherford’s gold foil experiment can be easily modelled. What you need: A ping pong ball, a hula hoop, string, sticky tape, straws, grains of rice It is best to complete this experiment outdoors. 1 Attach the string to the ping pong ball using sticky tape. Tie the other end of the string to the hula hoop so that when help upright, the ball is hanging in the centre of the hoop. FS 2 Get one person to hold the hula hoop vertically upright. Other students to stand approximately 1 metre away from the hula hoop. 3 Take some grains of rice and blow them through the straw at the hula hoop. O 4 Record your observations about when: a the rice grains didn’t hit anything PR O b the rice grains hit the ping pong ball c How often did the rice grains hit the ping pong balls? Try to express this as a percentage. E d You have just modelled Rutherford’s gold foil experiment. What did each component of your model represent? EC TE D The new model of the atom PA G e How successful was this model in representing Rutherford’s experiment? What were some inaccuracies in your model? N C O R R Ernest Rutherford changed the way we think about atoms. People had to get used to the idea that atoms were mainly empty space. How difficult would that have been, considering atoms make up substances such as solid steel? It certainly does not feel like space when you touch a steel bar. Rutherford’s model of the atom described the structure of an atom as containing three different subatomic (smaller than an atom) particles. U Rutherford’s model of the atom Imagine trying to persuade someone that what he or she thought was solid, like a plum pudding, was actually mainly space. You would need some pretty good evidence. Rutherford’s model was supported by the evidence, and was backed up with further studies of the structure of the atom. Rutherford’s model of the atom proposed the following: Figure 1.13 If you expanded one atom to the size of the Sydney Cricket Ground, the nucleus of that atom would still be no bigger than a pinhead. • Atoms are made up of two different subatomic particles. • The nucleus of an atom is made up of protons. • Protons carry a positive electric charge. 1.2 Subatomic particles 11 01_CRA_IS9_77563_TXT_LAY.indd 11 28/08/13 2:55 PM The nucleus containing positively charged protons Orbiting, negatively charged electrons • Electrons have a negative electric charge. It is important to remember that particles with opposite charges (one positive and one negative) will attract each other, and particles with the same charge (for example, two negative charges) will repel each other. The other important thing to know is that atoms are neutral – overall they have no electrical charge. In any atom there is always the same number of positive protons as negative electrons. The opposite charges cancel each other out and result in an overall neutral atom. Figure 1.14 A model of an atom of the element carbon as would have been proposed by Rutherford. PR O O Electron orbits • Electrons move around in the space outside the nucleus. FS • The mass of the atom is almost entirely due to the mass of the nucleus; electrons have very little mass in comparison. Remember PA G E Questions 1.2.1: Looking inside the atom 1 Explain, using the evidence described, why you think Rutherford concluded that: a the atom contained a lot of space b there was a central area of positive charge EC TE D 2 Describe an alpha particle. What was the most important new understanding of the structure of the atom that Rutherford inferred from his experiment with alpha particles? 3 In his model of the atom, how did Rutherford describe the electrons? 4 Name and describe three types of particle we now know are found inside the atom. R Apply 6 Carry out some research to find out more about Rutherford’s experiment. Identify and describe the technologies required for this experiment to be successful. U N C O R 5 Working with a partner, construct a three-dimensional model of an atom using modelling clay or other suitable materials. Make sure you label all parts correctly and identify which model of the atom you are representing. Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 12 28/08/13 2:55 PM Discovery of the neutron Figure 1.15 The improved version of Rutherford’s model of the carbon atom. The nucleus is composed of two subatomic particles: positive protons and neutral neutrons. The smaller negative electrons are found in orbits around the nucleus. PR O O FS Protons are all positive and are found in the nucleus of the atom. Rutherford theorised that there must be something to help the positive charges stay together. We know that positive charges would typically repel each other, but if they were held far enough apart they might just be able to stay together. Rutherford theorised that because an atom is neutral overall, these ‘things’ holding the positive protons apart must be neutral (as the electrons already balance out the protons). In 1932 Rutherford’s student, James Chadwick, proved the existence of the neutron. Chadwick was awarded the Nobel Prize in Physics in 1935 for his work in the discovery of the neutron. PA G Activity 1.2.2: How can you tell what is inside? E We now know that the atom contains three subatomic particles: neutrons (neutral), protons (positive charge) and electrons (negative charge). When it was first proposed, Rutherford’s model of the atom only contained protons in the nucleus. Protons and electrons are both charged particles, which means that studying them is somewhat easy as we can look at how they are affected by other charges. As a proton is positive, we can study the way it behaves when subjected to a positive or negative charge. It will repel positive charges and attract negative charges. The opposite occurs with the electron. But how do scientists study, let alone discover, a particle that is neutral? EC TE D This kind of investigation uses what scientists call ‘indirect evidence’. Many scientists have used indirect evidence when trying to work out what is inside the atom. What you need: ball, 2 nails, wooden block, 3 small boxes Form two teams (A and B) of three students to work with each other. 1 Team A places the ball in one of the boxes, the wooden block in another, and the two nails in the last box. The boxes are then closed. 2 Team B has to work out a way of knowing what is inside each of the boxes without opening or touching them. 3 Team B can then touch and examine the boxes, still without opening them. O R R • Was team B more successful at identifying what was inside the box when able to touch and examine the box? • How might scientists have used indirect evidence to model what is inside an atom? N C • Identify at least one other field of scientific investigation in which the scientists working in that field would have to use indirect evidence to develop their theories. U Questions 1.2.2: Discovery of the neutron Remember 1 Identify the scientist who discovered the neutron. 2 Explain why it was difficult to discover the neutron. 3 Copy and complete the following table: Name of subatomic particle Charge Location within the atom overmatter 1.2 Subatomic particles 13 01_CRA_IS9_77563_TXT_LAY.indd 13 28/08/13 2:55 PM Atoms and their masses + – neutron + proton – electron Figure 1.16 A lithium atom with mass number 7 and atomic number 3. – – + + + + + + + + – neutron + proton – – EC TE D – electron R Figure 1.17 An oxygen atom with mass number 16 and atomic number 8. Mass number C O R 13 6 N C Atomic number E U – PA G – – FS + O + PR O – neutrons in its nucleus. Carbon-13 has a mass number of 13. You will find out more about isotopes later. The mass of an atom is calculated by added the masses of all the protons and neutrons in its nucleus (remember that electrons are too tiny to bother counting). Protons have very slightly less mass than neutrons. The difference is so miniscule that to simplify the process, we assume that protons and neutron have the same mass on this scale, a mass of 1 amu. Therefore the mass of a whole atom in amu can be worked out by considering how many protons and neutrons the atom has. For example, a helium atom that contains 2 protons and 2 neutrons will have a relative mass of 4 amu. A carbon atom that contains 6 protons and 6 neutrons will have a relative mass of 12 amu. The atomic and mass numbers of an atom can be used to determine exactly how many protons, neutrons and electrons are in that atom. The atomic number is the number of protons. The mass number is the combined number of protons and neutrons. So the number of neutrons in an atom can be calculated by subtracting the atomic number from the mass number. Carbon-13 has a mass number of 13 and an atomic number of 6. This information tells us that a carbon-13 atom must have 6 protons and 7 neutrons (13 − 6 = 7). Remember that overall, all atoms are neutral. So for every positive proton in the nucleus, the atom must have the same number of negative electrons circling around the outside. Carbon-13 has 6 electrons to cancel out the charge of the 6 protons. Figure 1.18 A carbon-13 atom. The atomic number of 6 tells us there are 6 protons, which makes the element carbon. It is a neutral atom so there must also be 6 electrons. The mass number tells us the total number of protons and neutrons in the nucleus, so mass number − atomic number = number of neutrons. 13 − 6 = 7 neutrons. E – All atoms have a mass. The mass of an atom is made up mainly of the protons and neutrons in the nucleus of the atom. The mass of an electron is extremely – difficult to calculate because electrons are so small. Although electrons do have mass, they are so tiny that we don’t bother counting them when calculating – the mass of atoms. neutron Chemists have devised a relative + proton mass scale to use with atoms because – electron they are so small. It is called the atomic mass unit (amu), also known as the dalton (symbol Da). By definition, 1 amu is equal to one-twelfth the mass of a carbon-12 atom. This scale is more convenient – than using actual units (such as the gram). The number of protons in an atom determines – which element the atom is. If the number of protons inside the nucleus changed, the – neutron would also change. The element + proton number of protons in an atom is – electron called the atomic number. All atoms of the same element have the same atomic number. However, some atoms of the same element had different masses. They have the same number of protons, the same number of electrons, but different numbers of neutrons. These atoms are called isotopes. Carbon-12 is just one of the types of carbon atoms. It has 6 protons (like all carbon atoms) and 6 neutrons in the nucleus. It is the most common isotope of carbon. By convention, isotopes are represented with a dash after the element name (or symbol), E followed by the total number of protons and neutrons combined in the nucleus of the isotope (in this case, 12). The total number of protons and neutrons in an atom is called the mass number. For example, carbon-13 represents an isotope of carbon with 6 protons and 7 E N N P P P N N N N P P P N E E E 14 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 14 28/08/13 2:55 PM N u m e r ac y builder Calculating numbers of protons, neutrons and electrons One of the isotopes of iron, iron-56, has an atomic number of 26 and a mass number of 56. Calculate the number of protons, electrons and neutrons present in its uncharged atoms. FS Aluminium has one main isotope. It has an atomic number of 13 and a mass Your turn O Example number of 27. This means that it has 13 protons and 14 (27 – 13) neutrons in the nucleus. Its uncharged atoms will have 13 electrons. PR O For uncharged atoms, the number of electrons must equal the number of protons. If we know the atomic number and mass number of an isotope, we can calculate the number of protons, neutrons and electrons present in an uncharged atom of that isotope. Size is relative! PA G E In this case, we don’t really need to know how far each pace is, but we are able to compare the lengths of the throws. When referring to atoms, knowing the actual mass of atoms is not going to help us that much, partly because the mass is so small. But being able to compare the masses of different atoms is very important when investigating the behaviour of different atoms and elements. O R R EC TE D Relative scales are helpful in many situations where comparisons are made between objects or events. These scales are used when it is more important to know the differences between objects and events than the actual measurement (size, mass, time). The following conversation uses relative measurements: ‘Mum, Chloe has been in the shower for twice as long as I was.’ ‘I know, but you used three times as much shampoo as her!’ Another example of a relative scale is where something is chosen as a reference point to compare measurements. We can do this when we use paces to compare distances, for example, ‘I threw the ball 25 paces but he could only throw it 18 paces’. Deeper u n d e r s ta n d i n g N C Representing atoms U When it is important to show the number of particles within each atom, the method of representation shown in Figure 1.19 can be used. As a whole, the elements can be presented in a form called the periodic table. One of the most common types is shown in Figure 1.20. Key things to remember about the subatomic particles: • The atomic number of an atom is the number of positive protons in the nucleus; it determines the element of the atom. 1 Write down one example of another area of science or everyday life where relative units are used. 2 For your example, describe why you think relative units are used. 3 Outline the problems you think would arise when using relative units for measuring quantities. • Neutrons are also found in the nucleus but have no charge. • The mass number of an atom is the combined number of protons and neutrons. Mass number (total number of protons and neutrons) A X Z Symbol of element Atomic number (total number of protons) • Electrons are negatively charged. • An atom has the same number of electrons as protons. Figure 1.19 The conventional representation of an element. 1.2 Subatomic particles 15 01_CRA_IS9_77563_TXT_LAY.indd 15 28/08/13 2:55 PM 1 1 C H 12.01 1.01 Li 13 III A 5 Be 6.94 B 9.01 Lithium Beryllium 3 12 Na Mg 22.99 24.31 Sodium Magnesium 19 4 20 K Ca 39.10 40.08 Transition metals 3 III B 4 IV B 5 VB 6 VI B 7 VII B 8 9 VII BI 10 11 IB 12 II B 21 22 23 24 25 26 27 28 29 30 47.88 50.94 Sc Ti 44.95 V Cr 52.00 Mn Fe 54.95 5 Rb 38 39 87.62 88.91 Sr 85.47 40 Y Zr 91.22 41 Nb 92.91 42 Mo 6 7 56 Cs 137.33 Cesium Barium 87 88 Fr 57 to 71 Ba 132.91 Ra (223) 226.03 Francium Radium 72 58.93 58.70 63.55 65.39 Cobalt Nickel Copper Zinc 45 46 47 48 95.94 (98) Ru 101.07 Rh 102.91 Pd 106.4 89 to 103 19.00 20.18 Fluorine Neon 16 17 28.09 30.97 32.07 Al 26.98 31 Ga 69.72 49 In 114.82 118.71 Indium Tin 81 82 204.38 Thallium Unp (262) Unh (263) Uns (262) Uno (265) Une (266) Uun (267) 140.12 Lanthanum Cerium 89 Ac 227.03 90 Th 232.04 Actinium Thorium 59 Pr 140.91 60 Nd 144.24 61 Pm (145) Praseodymium Neodymium Promethium 91 92 231.04 238.03 Pa Protactinium U 62 Sm 150.4 63 Eu 151.97 64 Gd 157.25 65 Tb Ti Pb 207.2 Lead 158.93 66 Dy 162.50 67 Ho 164.93 Samarium EuropiumGadolimium Terbium Dysprosium Holmium 93 Np 94 Pu E 58 Ce 237.05 (244) 95 Am (243) S 33 As 74.92 51 Sb 18 Cl Ar 35.45 39.95 Chlorine Argon 34 35 Se 36 Br 78.96 Kr 79.90 Selenium Bromine 52 83.80 Krypton 53 Te I 54 121.74 127.60 Antimony Tellurium 83 Bi 208.98 84 126.90 131.29 Iodine Xenon 85 Po At (209) Xe (210) 86 Rn (222) Bismuth Polonium Astatine Radon Mass numbers in parentheses are from the most stable of common isotopes. 96 Cm PA G 57 La Rare earth elements Lanthanoid series 138.91 Sn 80 200.59 (261) 50 112.41 Mercury 108 72.61 Cadmium Gold 107 Ge 79 196.97 186.21 32 Silver Hg P Callium Germanium Arsenic 110 106 Si Aluminium Silicon Phosphorus Sulfur 195.08 Unq Actinoid series 16.00 Oxygen 15 Platinum 183.85 Metals Figure 1.20 A periodic table of the elements. 14.01 107.87 Au Ne Nitrogen 109 105 Pt 10 F 14 Iridium 180.95 Ir 9 O 12.01 192.22 104 Os 8 Helium Carbon 190.23 178.49 Re 78 4.00 17 VII A 13 Hafnium Tantalum Tungsten Rhenium Osmium W 75 Cd 7 N 16 VI A Boron 77 Ta 74 Ag 6 C 15 VA He 10.81 76 Hf 73 Zn 44 Rubidium Strontium Yttrium Zirconium NiobiumMolybdenum TechnetiumRuthenium Rhodium Palladium 55 Cu Iron 43 Tc Ni 55.85 Potassium Calcium Scandium Titanium Vanadium ChromiumManganese 37 Co 14 IV A PR O 11 2 Non-metals 4 3 2 Carbon 2 II A Hydrogen 18 VIII A Atomic number Chemical symbol Atomic mass Name of element 6 FS New designation Original designation O 1 IA (247) 97 Bk (247) 98 Cf (251) 99 Es (252) 68 Er 69 71 70 Tm Yb 167.26 168.93 Erbium Thulium Ytterbium Lutertium 100 Fm (257) Uranium NeptuniumPlutonium Americium Curium BerkeliumCaliforniumEinsteinium Fermium 173.04 Lu 101 174.97 102 103 (258) (259) (260) Mendelevium Nobelium Lawrencium Md No Lr Questions 1.2.3: Atoms and their masses EC TE D Remember 1 Copy and complete the following table, showing the numbers of subatomic particles in a range of atoms. Atom name and symbol Number of protons 40 20 Fluorine (F) 9 19 9 Sodium (Na) 11 R 20 O R C N Mass number Calcium (Ca) Argon (Ar) U Atomic number 11 40 Sulfur (S) Number of neutrons Number of electrons 20 20 12 18 16 16 a Explain how you were able to calculate the number of neutrons in the argon atom. b Explain how you were able to work out the atomic number and the mass number of the sulfur atom. 2 Identify the subatomic particle not found in the nucleus of the atom. Apply 3 Imagine all the elements did not have names and were simply identified by their atomic numbers. Explain if you think this would make chemistry easier to understand. What problems do you think this would cause? What would be the advantages? overmatter 4 Identify how many electrons there are in the nitrogen atom, which has an atomic number of 7 and a mass number of 14. 16 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 16 28/08/13 2:55 PM Atomic mass and isotopes + + U N C – + 44.95 47.88 Scandium Titanium 39 40 50.94 Zr 91.22 Zirconium 57 to 71 Hf + ++ 54.95 42 43 Mo 92.91 Tc 95.94 (98) 27 Fe 73 74 Ta Re 29 Cu 30 Zn 58.93 58.70 63.55 65.39 Iron Cobalt Nickel Copper Zinc 44 45 46 47 Ru Rh 101.07 75 W Ni 55.85 Pd 102.91 Niobium MolybdenumTechnetium Ruthenium Rhodium 72 28 Co 76 Ir Ag 48 Cd 106.4 107.87 112.41 Palladium Silver Cadmium 77 Os 78 Pt 79 Au 80 Hg 178.49 180.95 183.85 186.21 190.23 192.22 195.08 196.97 200.59 Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury 104 105 106 107 108 109 110 111 Rf Db (205) 105 Sg Bh (271) Hs (272) Rutherfordium Dubnium Seaborgium Bohrium Mt (277) (276) (281) Rg (280) 112 Cn (285) Figure 1.21 Some atomic numbers and atomic masses in the periodic table. PA G E carbon, have the same number of protons, they may well belong to a group of carbon atoms that have a different numbers of neutrons. Isotopes Ds Hassium Meitnerium Darmstadtium Roentgenium Copernicium PR O 89 to 103 52.00 Nb 88.91 26 Mn Vanadium Chromium Manganese 41 Yttrium 25 Cr FS Y 24 V For many of the elements, the number of neutrons in the atoms can vary. For example, most carbon atoms have six neutrons in their nucleus but some have seven and some have eight. The different forms of the atoms of an element with different numbers of neutrons are called isotopes. Carbon-12 (6 protons and 6 neutrons) is the most common isotope form of carbon atoms in the natural world (about EC TE D O R – 6 neutrons 23 Ti – R – 22 21 Sc O On the periodic table you will have seen the atomic masses of the elements listed. These masses are not whole numbers and are not the same as the mass numbers of the atoms (although they are pretty close). They are a more accurate way of comparing the masses of the atoms of different elements. But why are many of them not whole numbers? We certainly cannot have part of a proton or part of a neutron in an atom. Electrons do have some mass, but not really enough to make much difference to the overall mass of the atom. So where do these atomic masses come from? Generally, not all the atoms within an element have the same mass. This is because they are not identical. Why is this? What do they have in common and what is different? All the atoms of an element have the same unique number of protons, their atomic number, sometimes shown by the letter Z. The atomic number is used to identify the element. For example, all carbon atoms contain 6 protons in their nucleus, so their atomic number is 6. If you examine the periodic table of the elements you can see that the elements are listed in order of their atomic number. However, although all the atoms of one particular element, such as 6 Protons – 8 neutrons – + + – 6 Protons + – + + + – – – – 12 14 carbon-12: C carbon-14: C 6 6 – – 7 neutrons – 6 Protons + + + – + + + – – 13 carbon-13: C Figure 1.22 The three isotopes of carbon. 6 1.2 Subatomic particles 17 01_CRA_IS9_77563_TXT_LAY.indd 17 28/08/13 2:55 PM FS chemists use the average mass of the isotopes of the element for calculations. This average mass is known as the relative atomic mass of the element. Because almost all carbon atoms exist as the carbon-12 isotope and only a very small proportion are present as the two heavier isotopes, the relative atomic mass is only just above 12. The relative atomic masses of the elements are usually shown in the periodic table, correct to one or two decimal places. Be careful not to mix this up with their atomic numbers or the mass numbers shown in isotopes. L i t e r ac y bu i l d e r PR O O 98.9%). Only 1.1% of natural carbon on the Earth is carbon-13 atoms (6 protons and 7 neutrons). You may have heard of carbon-14. This is a very small percentage of total carbon and is comprised of radioactive carbon-14 atoms. A carbon-14 atom breaks down naturally by radioactive decay to form an atom of the element nitrogen, an electron and a release of electromagnetic radiation. You will learn more about radioactivity in section 1.3 of this chapter. Most elements have more than one naturally occurring isotope. In these cases, Formation of atoms E as nuclei of new atoms are created. The process of atom formation is not a gentle, well-ordered process. It is therefore not surprising that the products of these interactions are not all identical. In your research into nucleosynthesis, try to identify atoms that: • were produced very soon after the Big Bang EC TE D PA G Why do some elements have atoms with different numbers of neutrons? Why are they not all the same? To answer this question you might want to investigate the idea of nucleosynthesis, the process by which atoms are formed. This may have been in the early days of the Universe, in massive stars or in supernova explosions. The processes involved in the formation of atoms require huge amounts of energy; they are powerful, often explosive events where neutrons and protons undergo violent interactions • were produced later in the history of the Universe • are unstable and rapidly convert into more stable atoms. R Questions 1.2.4: Atomic mass and isotopes Remember U N C O R 1 Explain the meaning of ‘mass number’ and how this name arose. Use an example to assist your explanation. 2 Explain why the atomic number of an element is always a whole number but the relative atomic mass of an element is often not a whole number. Apply 3 A student wrote that all the atoms of an element are identical. Is this correct? Explain your answer. 4 Using your knowledge of isotopes and a copy of the periodic table, copy and complete the following table: Isotope symbol Isotope name Atomic number of element Number of protons Number of neutrons Number of electrons in uncharged atom U 238 92 Oxygen-16 overmatter 18 Oxford Insight SCIENCE 9 Australian Curriculum for30 NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 18 10 20 36 29 34 28/08/13 2:55 PM Arranging electrons valence shell. The number of electrons in the valence shell of an atom makes a big difference to the chemical properties of the element, and will affect how the atom will bond with other atoms. Oxygen: Table 1.1 Electron configurations for electron shells of an atom. • There are now 18 electrons left to place in shells. Remember, the second shell can only hold 8 electrons. First 2 Second 8 Third Up to calcium: 8 Above calcium: 18 Fourth 32 O • Second shell holds the other 6 electrons. Calcium: Figure 1.23 Niels Bohr E • Atomic number of calcium is 20, so the uncharged atom contains 20 electrons. • First shell can only hold 2 electrons. – – – – – – a • There are now 10 electrons left to place in shells. Remember the third shell can only hold 8 electrons. – – • Fourth shell holds the last 2 electrons. b R – – – – – – – – – – – – – 2, 8, 8, 2 b Calcium 2, 6 a Oxygen – – – – – – – – N C O R • Calcium’s electron configuration is written as 2.8.8.2. – For the first 20 elements, the third shell can only hold eight electrons. For atoms with atomic numbers greater than 20, the third shell can accommodate up to 18 electrons. In Year 9, you only have to – – consider electron configurations for the first 20 elements (up to calcium). – – Bohr also stated that the electrons of an atom are normally located as close to the nucleus as possible, because this is a lower energy state and is more stable. Therefore, the shells are filled from the inside out. The electron configurations of oxygen and calcium are compared in Figure 1.24. – – 2, 6 Electron configurations are often Oxygen represented by shelladiagrams, which show the electron shells as circles and the electrons in pairs. The outermost occupied shell of uncharged atoms is known as the U PR O • Oxygen’s electron configuration is written as 2.6. PA G Maximum number of electrons in shell • First shell can only hold 2 electrons. EC TE D Electron shell • Atomic number of oxygen is 8, so the uncharged atom contains 8 electrons. FS After Rutherford had refined his model of the atom, another scientist named Niels Bohr concluded that the electrons in an atom do not behave quite like the planets around the Sun. Instead, he proposed that they move about the nucleus in circular orbits at certain distances from the nucleus. The more energy they have, the further their orbit is from the nucleus. These sets of orbits are known as electron shells. There is a limit to the number of electrons that can be found in any of the shells. This is called the Bohr model of the atom. The arrangement of electrons in an atom is called its electron configuration. In this book we will consider the electron configuration of an atom according to the Bohr model of the atom. Table 1.1 shows the number of electrons each shell can contain. Figure 1.24 Electron configurations for (a) oxygen and (b) calcium can be shown as simple shell diagrams. 1.2 Subatomic particles 19 01_CRA_IS9_77563_TXT_LAY.indd 19 28/08/13 2:55 PM – nucleus + electron jumps up one energy level – FS PA G energy level 1 the ground state O absorb electromagnetic radiation PR O Many substances give off coloured light when small samples are introduced into a flame. When this light is seen through a spectroscope (an instrument that breaks the light up into its colours), a pattern of coloured lines is observed. This pattern is known as an emission spectrum and is unique for each element. Bohr proposed that when atoms of the elements were given energy in a flame, the electrons jumped from their normal shell to one further out from the nucleus. He described the electrons as being excited. Because this higher energy state was unstable, the electrons then jumped back to their normal levels almost instantly. For each electron jump, a certain amount of energy was emitted (given out) in the form of light of a particular wavelength. Each coloured line in the spectrum represents one of these wavelengths of light. The possible values of energy for the electrons present are slightly different for each element, so each element produces a different spectrum. The emission spectrum is like the ‘fingerprint’ for an element as each element produces a unique spectrum. This method has been used to find out what elements are present in stars, millions of kilometres away from us here on the Earth, because light from these stars can be analysed using a spectroscope. Although we can never see electrons, we can observe the energy changes caused as electrons move between the different shells in an atom. This energy is often in the form of light, and different atoms can produce different colours of light. This effect can be used to distinguish between different atoms, and therefore identify different elements. This is how flame tests work. E Evidence for electron shells EC TE D energy level 2 Figure 1.25 As electrons jump from shell to shell they emit electromagnetic radiation in the form of light. 4.58 4.09 3.03 x10-28J N C O R R energy U Figure 1.26 The three electrons found in this atom will absorb energy to jump into the excited state. As they come back down to the ground state, they will release the energy in the form of a photon, or light. The ‘jumps’ back to their ground state, depending on where they were in the excited state, will release different bands of light. The bigger the jump, the more energy released. 4th–>2nd 3rd–>2nd 5th–>2nd 20 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 20 28/08/13 2:55 PM Experiment 1.2.1: Flame tests With your understanding of emission spectra, predict what you think might happen when you burn samples of different metal salts in a Bunsen burner hot flame. Use your prediction to formulate a hypothesis for this experiment. Wear safety goggles. Ensure hair is tied back and loose clothing is removed or tucked out of the way. Wire loops and flames are hot. Be careful not to burn yourself. 1 M hydrochloric acid can give a small chemical burn. Wash it off skin with tap water immediately. FS >> >> >> >> O WARNING Aim To observe the coloured light emitted when certain substances are heated in a flame. carbonate, barium sulfate, calcium carbonate and strontium carbonate • Wire loops • Hydrochloric acid (1 M) E • Bunsen burner • Heatproof mat • Solid samples of sodium carbonate, copper carbonate, potassium PR O Materials PA G Method 1 Set up your Bunsen burner, observing safety instructions, and light your Bunsen burner on the safety flame. EC TE D 2 Adjust your Bunsen burner to the blue flame. Take a wire loop and dip it in a small beaker of 1 M hydrochloric acid. Flame the loop (heat it briefly in the blue flame). This will clean the loop, ready for your solid sample. 3 Take a loop of solid chemical and place it in the flame. Observe the colour of the flame. Try not to lose the solid down the Bunsen burner barrel. This could block the burner and contaminate the flame, changing the colour. 4 Once you have finished your observation, dip the loop in the 1 M hydrochloric acid again and re-flame it. This will clean the loop for the next sample. Chemical O R R Results Include your results in a table. Flame colour Sodium carbonate C Copper carbonate N Potassium carbonate U Strontium carbonate Discussion 1 Explain why the loop was treated with hydrochloric acid before any carbonates were tested. 2 Explain what you think caused the flame to change colour. 3 Describe the kind of change that caused the flame colour to change. Was it chemical or physical? Identify the evidence that supports your answer. 4 Discuss why electrons in different elements produce different colours. 5 Does the metal or the carbonate part of the powder cause the colour change? Identify the evidence that that supports your answer. 6 Using Figure 1.26 to help, explain how the subatomic structure of an atom can overmatter 1.2 Subatomic particles 21 01_CRA_IS9_77563_TXT_LAY.indd 21 28/08/13 2:55 PM from his burner, the glass made the flame turn yellow. He then started to investigate other chemicals and recorded the colours that were produced. This work led other scientists to develop more accurate methods of analysis by using coloured spectra from different substances. Although the Bunsen burner seems like a simple piece of equipment, it is a good example of how developments in technology can open up possibilities for scientific discoveries. FS The German chemist Robert Bunsen was very particular about the quality of the equipment he used. When he took over as professor of chemistry at the University of Heidelberg, he insisted on having new laboratories built. However, he still was not happy with the burners used at the time because they all produced yellow or smoky flames. In 1855 he invented a burner that produced an almost invisible flame. When melting glass to make laboratory equipment, he noticed that when heated with the colourless flame O Figure 1.27 Robert Bunsen The Bunsen burner Questions 1.2.5: Arranging electrons Remember PR O Deeper u n d e r s ta n d i n g E 1 In the Bohr model of the atom, identify the maximum number of electrons the second electron shell can contain. PA G 2 Propose a reason the second shell can contain more electrons than the first shell. Apply 3 A potassium atom contains 19 protons. EC TE D a How many electrons will be present in a potassium atom? Explain your answer. b Describe the electron configuration of a potassium atom according to the Bohr model. c Identify how many electrons are in the valence shell of a potassium atom. d Describe what could be done to potassium atoms to make electrons jump into the fifth shell. R 4 Copy and complete the table. O R Element Chemical formula Atomic number Electron configuration Helium U N C Carbon 6 Neon 2.8 1 Magnesium 17 2.8.3 5 Identify a piece of laboratory equipment (other than the Bunsen burner) and explain how it has made a significant impact on scientific discoveries. 6 Identify the element you think caused the yellow colour that Bunsen saw when he was heating glass. 22 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 22 28/08/13 2:55 PM Atoms and ions magnesium ion Mg2+ (2, 8)2+ gains 1 electron U N C O R R magnesium atom Mg 2, 8, 2 EC TE D loses 2 electrons E PR O O FS electrons leading to a charged atom can only be caused by the loss or gain of electrons. Ionisation may occur when atoms come together to form chemical bonds. It can also occur when atoms are exposed to radiation. When ions are formed, the electrons in the outer electron shell (the valence shell) are affected. When ions are formed, normally the resulting ion has a full outer shell of electrons because it is a stable arrangement. In this situation the first three shells are full, with 2, 8 and 8 electrons respectively. PA G Atoms are neutral. The amount of negative charge within the atom is the same as the amount of positive charge. The number of protons (positive) is always the same as the number of electrons (negative). However, if electrons are lost or gained from the outside of the atom, this balance is disturbed – there will no longer be the same number of protons as electrons and the atom becomes charged. Charged atoms are called ions, and the formation of ions is called ionisation. Protons cannot be removed from the nucleus so an imbalance of protons and chlorine atom Cl 2, 8, 7 Figure 1.28 shows the formation of magnesium and chloride ions. The magnesium atom originally had 2 electrons in its valence shell. It will lose both of these electrons (it is easier to lose 2 than to gain Figure 1.28 How magnesium and chloride ions are formed. chloride ion Cl– (2, 8, 8)– 6), so the second shell is now the valence shell and will be full. The chlorine atom had 7 electrons in its outer shell. It would gain 1 electron to make this outer shell full with 8 electrons (it is easier to gain 1 than to lose 7). 1.2 Subatomic particles 23 01_CRA_IS9_77563_TXT_LAY.indd 23 28/08/13 2:55 PM Calculating ion charge When ions are formed, protons are held in the nucleus and are not affected by changes occurring outside the nucleus. The number of protons stays the same. Electrons are negatively charged, so for each extra electron gained the charge on the atom becomes negative by one. If an electron is lost, the charge on the atom becomes positive by one as there will now be extra protons compared to the number of electrons. Table 1.2 gives some examples. Table 1.2 Some positive and negative ions. Electron configuration of ion Change Oxygen (O) 2.6 2.8 gained 2 electrons Chorine (Cl) 2.8.7 2.8.8 gained 1 electron Sodium (Na) 2.8.1 2.8 Calcium (Ca) 2.8.8.2 2.8.8 Remember −2 Name and formula of ion Oxide (O2−) Chloride (Cl−) lost 1 electron +1 Sodium (Na+) lost 2 electrons +2 Calcium (Ca2+) E PR O −1 PA G Questions 1.2.6: Atoms and ions Charge of ion FS Electron configuration of atom O Name and symbol of atom 1 Refer to Table 1.2 and explain the patterns you notice about the: a names of the negative ions EC TE D b electron configuration of the ions c differences between the metals and non-metals Apply 2 Predict the charges on the following ions: a potassium (atomic number 19) R b aluminium (atomic number 13) c nitride (produced from nitrogen atoms with atomic number 7) U N C O R 3 The elements neon (atomic number 10) and argon (atomic number 18) do not normally form ions. Suggest why. 24 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 24 28/08/13 2:55 PM 1.2 Subatomic particles Remember and understand 1 Identify the location of the following particles in an atom and state their charges. a Identify the atomic number of the element. [1 mark] b Identify what element it must be. [1 mark] Checkpoint 3 Explain the difference between an atom and an element. Give an example to support your answer. [2 marks] 4 Explain why the mass numbers of isotopes are exact whole numbers but the relative masses of most elements are not. [2 marks] O 9 Tellurium, element 52, has a relative atomic mass of 127.6. The next element, iodine, has a relative atomic mass of 126.9. a Write the symbol for the isotopes of tellurium-127 and iodine-127. [2 marks] b Explain why the atoms of these two different elements can have the same mass number. [2 marks] 10Using the example of atoms, explain the difference between a model and a theory. [2 marks] EC TE D 5 Titanium, element 22 in the periodic table, has five naturally occurring isotopes. Describe what the isotopes of titanium have in common and in what way(s) they are different. [2 marks] Analyse and evaluate PR O 2 When an atom is uncharged, identify the number of protons and electrons present. [1 mark] E c electron [2 marks] PA G b neutron [2 marks] FS c Identify the electron configuration of the next element on the periodic table. State your reasoning. [2 marks] a proton [2 marks] Apply R U 6 235 is a naturally occurring radioactive 92 isotope of uranium used in nuclear reactors. In an uncharged atom, identify how many of the following are present: O R a protons [1 mark] b neutrons [1 mark] C c electrons [1 mark] U N 7 Only 0.7% of the uranium atoms in naturally occurring uranium exist as uranium-235. The other isotopes present are uranium-234 (0.01%) and uranium-238 (99.3%). Design an appropriate table to compare the symbols, atomic numbers and mass numbers of these isotopes. [3 marks] 8 According to the Bohr model of the atom, the uncharged atoms of a particular element have the electron configuration of 2.8.8. 11Scientists have used indirect evidence to infer what it is like inside the atom, in the same way that astronomers have worked out the temperature and composition of stars. List the advantages and disadvantages of using indirect evidence to develop theories in science. [2 marks] Critical and creative thinking 12Create a poster or digital presentation to show different models of the atom, from John Dalton’s original solid sphere theory to the Bohr model used today. Use the Internet to find images of the scientists involved and place copies onto your poster. Identify the year in which each model was proposed and include a scaled timeline. [5 marks] Making connections 13Apply your understanding of atoms to suggest reasons for the following: overmatter TOTAL MARKS [ /40] 1.2 Subatomic particles 25 01_CRA_IS9_77563_TXT_LAY.indd 25 28/08/13 2:55 PM E When changes like this happen, the atom is said to decay. Uranium is an example of a radioactive material because it sometimes contains these unstable atoms and will decay over time. Radiation can be emitted from the nucleus of radioactive atoms as they decay. The most common types of radiation emitted are alpha (α), beta (β) and gamma (γ) radiation. These three types of radiation are different, but are all caused by the nucleus of the atom breaking up. Why does this happen? Some atoms contain nuclei that become more stable by giving out radiation. EC TE D PA G Atoms are usually around for a long time. An oxygen atom formed billions of years ago will still exist today in a piece of iron ore (Fe2O3), in a glass of water (H2O), or even as a carbohydrate (such as glucose, C6H12O6) in your body. However, some atoms are not so stable and can change over time. As they change, the subatomic particles may separate and the combination of protons, neutrons and electrons that defined that atom may change. These changeable atoms are radioactive. This change can happen quickly, or it might take tens, thousands or even millions of years – it all depends on the type of atom. O What is radioactivity? FS With an understanding of the structure of atoms, the idea of radioactivity can be explained. Alpha (α), beta (β) and gamma (γ) are different types of radiation released from atoms. When does this happen? Which atoms are radioactive? What makes these atoms different from other atoms? Examining the structure of the nucleus of these atoms can help answer these questions. In-depth knowledge of atoms has allowed scientists to understand the risks of radioactivity and use radioactive materials for areas such as medicine. PR O 1.3 Radioactivity R Activity 1.3.1: Exploring uranium U N C O R Australia has 23% of the world’s known uranium reserves. Uranium is the main fuel used in nuclear power stations. The atomic number of uranium is 92, which means that all uranium atoms contain 92 protons. The most common form is uranium-238 (238U) with more than 99% of natural uranium in the world being this isotope. Most of the rest is uranium-235 (235U). 1 Using your knowledge from section 1.2, calculate the number of protons and neutrons in an atom of uranium-238. Repeat the calculation for uranium-235. Like all uranium isotopes, uranium-235 is an unstable atom and can decay into other atoms. This contributes to uranium’s classification as a radioactive element. When uranium-235 decays, it can decay into a different atom, thorium-231. Thorium has an atomic number of 90. 2 Calculate the number of protons, electrons and neutrons in an atom of thorium-231. 3 Identify the number of protons a uranium atom loses to become thorium-231. Figure 1.29 The Ranger Uranium Mine in Kakadu National Park, Northern Territory. 4 Identify the number of neutrons a uranium atom loses to become thorium-231. 5 Identify the atom that contains the number of protons and neutrons in questions 3 and 4 in its nucleus. 26 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 26 28/08/13 2:55 PM You should have found that as the uranium-235 atoms decay to the thorium-231 atoms, the equivalent of the nucleus of a helium atom is lost. This particle is called an alpha particle and these particles are released from the original uranium atom with enough energy to travel away from the atom. 6 Research alpha particles and outline: a how they can be represented b what electric charge they possess c how dangerous they are FS d how they can be stopped Marie Curie PR O Figure 1.30 A Geiger counter is used to detect radiation. Deeper u n d e r s ta n d i n g U N C O R R EC TE D Marie Curie was an exceptional scientist. She was born Maria Sklodowska in Warsaw, Poland. When she married Pierre Curie, a professor of physics at Sorbonne University in Paris, she became Marie Curie. In 1903 the Nobel Prize in Physics was awarded to Marie Curie, Pierre Curie and Henri Becquerel for their work in discovering radioactivity. Marie Curie was the first woman to ever receive a Nobel Prize. After her husband died in a tragic horse-drawn vehicle accident, she took over his role in the university as the chair of physics. In 1911 Curie received her second Nobel Prize, this time for Chemistry, making her the first person to win or share two Nobel Prizes. The dangers of radiation were not understood at the time and Curie’s pioneering work in radiation eventually led to her own death. She died of a fatal form of anaemia (a disease involving blood cell deficiencies) brought on by her exposure to radiation over years in the laboratory. Even her original laboratory reports are still considered too dangerous to handle and are kept in boxes lined with lead to shield people from the radiation the reports emit. One year after Marie Curie’s death in 1934, her daughter, Irene, shared another PA G E Radiation that exists all around us from various sources is called background radiation. Radioactive minerals can be found in the ground, and radiation in the form of cosmic rays comes from the Sun and space. All this radiation can be measured with a Geiger counter. O Detecting radiation Figure 1.31 Marie Curie. Nobel Prize in Chemistry for her work, in collaboration with her husband, on artificial radioactivity. Her grandchildren are well-known scientists, too, with her granddaughter Hélène Langevin-Joliot being a professor of nuclear physics, and her grandson Pierre Joliot being a wellknown biochemist. Marie Curie’s research led to a large number of other scientific discoveries. Rutherford’s experiments into the structure of the atom would not have been possible without the discovery of radioactive materials, which Rutherford used as a source of the alpha particles. 1.3 Radioactivity 27 01_CRA_IS9_77563_TXT_LAY.indd 27 28/08/13 2:55 PM Isotopes and radioactive decay FS O Whether a nucleus is stable depends on the number of neutrons and number of protons in the nucleus. There is no easy way to predict the stability of different atomic nuclei. Some nuclei are very stable, such as carbon-12 nuclei with 6 neutrons and 6 protons. However, carbon-14 with 8 neutrons in its nucleus is less stable and will decay over time, giving out radiation to form a different atom, nitrogen-14. Scientists know how long this process takes, therefore carbon-14 isotope decay can be used to measure the age of objects, including matter from living things. This method is called carbon dating, and is the most common way of dating ancient artefacts, and plant and animal material. Carbon dating was used to measure the age of the Shroud of Turin, a linen cloth believed by many to be the cloth that covered Jesus’s face after his crucifixion and burial. The decay of radioactive isotopes is often a very slow change. One gram of carbon-14 today would take more than 5000 years until half of it had decayed and become only 0.5 grams. The remaining 0.5 grams would take more than another 5000 years to get down to 0.25 grams, and more than another 5000 years to reduce to 0.125 grams. After ten half lives (or 50 000 years), a gram of carbon-14 PR O Carbon dating would have decayed to leave about 0.0001 grams of carbon-14. Unless the amount of carbon-14 is measured over a very long period, it might seem that no change is occurring. However, scientists using extremely sensitive instruments are able to detect the radiation being released during the decay process. Some radioactive atoms decay incredibly quickly. In a sample of the isotope lithium-8, half of it decays in less than 1 second. The problem for lithium-8 would not be trying to detect how it is U N C O R R EC TE D PA G Deeper u n d e r s ta n d i n g E Figure 1.32 Some smoke detectors contain a radioactive source. Earlier in this chapter you learned about isotopes. Hydrogen, for instance, has three isotopes: hydrogen-1 (1 1 H), hydrogen-2 (2 1 H) and hydrogen-3 (3 1 H). Each isotope of hydrogen has one proton (the same atomic number) but different numbers of neutrons (different mass numbers). In some isotopes, when the ratio of neutrons to protons becomes too high, the nucleus is unstable and it decays or changes into another isotope. This is known as radioactive decay, and causes the emission of radiation. Hydrogen-1 and hydrogen-2 are stable, but hydrogen-3 is unstable and emits a beta particle (a single electron). Hydrogen-3 is therefore a radioactive isotope called a radionuclide. Radionuclides can occur naturally or they can be produced in a nuclear reactor. Figure 1.33 Carbon dating has indicated that the Shroud of Turin is less than 2000 years old. 28 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 28 28/08/13 2:55 PM 1 Suggest a reason why carbon-12 atoms are more stable than carbon-14 atoms. 2 Describe the dangers of isotopes that decay very quickly. 3 Identify the dangers of radioactive isotopes that decay very slowly. 4 When used in medicine, radioactive isotopes that decay relatively quickly are used. Explain why this might be. O FS 5 Carbon dating works because, as isotopes decay, the amount of radiation released reduces over time. Therefore the measured level of radiation will indicate the age of the object. PR O a Outline why atoms that decay extremely slowly are not generally be used for dating objects. E b Outline why atoms that decay very quickly are not generally used for dating objects. PA G changing, but actually detecting it at all. Carbon-14 is by no means one of the slow ‘decayers’. Uranium-235, a radioactive isotope of uranium, would take 700 million years to reduce to half of what you started with. If you had some uranium-238, a nonradioactive isotope, most of it would still be there four billion years later! In science, there are many situations where change takes place over a range of time scales. Radioactive decay is special as it is a purely random process. It is impossible to predict how long a single particular atom will take to decay, giving out radiation as it does so. With billions of atoms in any one sample, the overall rate of decay can be predicted. Think about a glass of water evaporating: it is impossible to predict when one particular water molecule will escape from the liquid, but overall we can predict how long the water will take to evaporate. Questions 1.3.1: What is radioactivity? Remember a ‘isotope’ EC TE D 1 Explain the meaning of each of the following terms: b ‘radioactive decay’ c ‘radionuclide’ 2 Recall an isotope of uranium that is radioactive. 3 Explain why radioactive decay occurs. Research O R R 4 Recall the name of an instrument that can be used to determine the level of radioactivity. 5 Identify at least two achievements of Marie Curie. N C 6 Different parts of the world have different levels of background radiation. Conduct an Internet search to find out how background radiation is measured and what the levels are in some typical places. U 7 Find out information about certain jobs that expose workers to more radiation than what is considered ‘typical’. How is radiation exposure controlled in these jobs? 8 Research the development of ideas about the nature of radioactivity. Present your information as an annotated time line. Include the relevant dates, scientists involved as well as their key ideas. Include any significant technological developments that may have contributed to the new discoveries. 1.3 Radioactivity 29 01_CRA_IS9_77563_TXT_LAY.indd 29 28/08/13 2:55 PM Types of nuclear radiation O FS but the proton remains in the nucleus, increasing the atomic number of that atom by one. Changing the number of protons in the atom changes the element of that atom. It is now known that another particle, called an antineutrino, is also released in this process. Gamma rays are high-energy electromagnetic rays similar to X-rays. They are emitted after alpha particle or beta particle emission, when the nucleus is still excited. An example is when cobalt-60 decays to form nickel-60: PR O Alpha (α), beta (β) and gamma (γ) radiation all originate from an unstable nucleus. An alpha particle is identical to the nucleus of a helium nucleus. It contains two protons and two neutrons. Americium-241 (commonly used in smoke detectors) is an example of an alpha particle emitter. It decays to neptunium-237, a more stable isotope. The decay of americium-241 to neptunium-237 can be shown as: 237 93 Am Np PA G 241 95 E Alpha particle The number of protons and neutrons shown in the diagram are not exact. A p + 4 α 241 m → 237 95 2 93 N C O R R EC TE D The mass numbers on each side of the arrow add to 241, which demonstrates that the total mass of the particles before and after the decay is the same. Beta particles are produced when a neutron in the nucleus decays into a proton and an electron. The electron is the beta particle that leaves the atom. An example of beta decay is the decay of carbon-14 to nitrogen-14: U N Beta particle 14 6 C 14 7 N 0 C → 14 14 6 7 N + −1 β The beta particle has very little mass, so the new nucleus formed has a mass very similar to the original carbon-14 nucleus. The electron is formed when a neutron breaks apart into a proton and an electron. The electron is emitted from the atom, 60 27 Co 60 28 Beta particle + Ni Cobalt-60 is an artificially produced radioisotope used in medical radiotherapy, sterilisation of medical equipment and irradiation of food. Because gamma radiation is an electromagnetic wave, rather than a particle like alpha and beta radiation, gamma radiation is highly penetrating and can cause cell damage deep within the body if exposure levels are high. Radioactive half-life Radioactive decay is a random process and we cannot predict which radioactive atom nuclei in a sample will decay at any given moment. However, the rate of radioactive decay follows a pattern. As a radioactive sample decays, less and less of the original substance is left and the radioactivity drops. The half-life of a radioactive material is the time taken for half of the radioactive nuclei in a sample to decay, resulting in the radioactivity of the original radioactive material to drop to half of what it was. When the radioactivity reaches one-half of its original level, one half-life has passed. 30 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 30 28/08/13 2:55 PM Bismuth-213 46 minutes Technetium-99m 6 hours Lutetium-177 6.7 days Iodine-131 8 days Chromium-51 28 days Strontium-89 50 days 25 0 0 25 50 75 100 125 150 175 Time (years) Figure 1.34 A radioactive decay curve for strontium-90, which has a half-life of 28.8 years. FS Half-life 50 O Radionuclide 75 Activity 1.3.2: Modelling radioactive decay PR O Table 1.3 Half-lives of important medical radionuclides. 100 Strontium-90 (g) When it reaches one-quarter of its original level, two half-lives have passed and the pattern continues. A graph of radioactive decay against time gives a characteristic shape called an exponential decay curve. EC TE D 1 Create a table to record your results. PA G E You can model the idea of exponential decay and half-life by using M&M’s to represent the nuclei of atoms. Predict the outcome of the activity. Will your model accurately predict radioactive decay? What you need: pack of M&M’s, A4 plain paper, disposable plastic cup 2 Record the total number of M&M’s, then place them into the plastic cup. 3 Shake the cup and tip all the M&M’s onto the paper. 4 Those that have the ‘M’ facing upwards represent atoms that have decayed. Move these to one side to form a ‘discard’ pile. 5 Count the remaining ‘nuclei’ and record the number in your table. 6 Place these remaining nuclei back into the cup, shake them and tip out again. R 7 Remove the decayed nuclei to the discard pile and count those remaining. O R 8 Continue until you have three or fewer nuclei. 9 Repeat the whole process two more times so you have a total of three trials. N C 10Draw a set of axes with the number of atoms remaining (vertical axis) and the number of shakes (horizontal axis). Using just this set of axes, plot points and draw a line or curve of best fit through the points for each of the three trials. U • In this activity, the atomic nuclei were represented by M&M’s. What represented the half-life of the decay process? • Are the shapes of the three curves similar or different? Comment on your answer. • Do you think the overall shape of the curves would be different if you started with more atomic nuclei? Explain your answer. • In this activity, could you predict when each individual nucleus would decay? Why is this similar to the behaviour of real radioactive atoms? • In this activity, you would eventually end up with no ‘undecayed’ M&M’s. Would this be the case with a real radionuclide? Explain your answer. 1.3 Radioactivity 31 01_CRA_IS9_77563_TXT_LAY.indd 31 28/08/13 2:55 PM N u m e r ac y bu i l d e r Calculating half-life Answer Your turn Calculate the count rate of a sample of iodine-131 after 32 days if the initial count rate is 2016 counts per minute. The halflife of iodine-131 is 8 days. FS The half-life of technetium-99m is 6 hours. If the initial radioactive count rate is 1088 counts per minute, what will be the count rate after 30 hours? O Example rate is halved. So: 1st half-life = 544; 2nd half-life = 272; 3rd half-life = 136; 4th half-life = 68; 5th halflife = 34 Therefore the answer is 34 counts per minute. PR O If you know the half-life of a material, you can calculate how much of a material is left after a specific time. Likewise, if you know the rate of decay of a material, you can calculate its half-life. PA G Effects of radiation E Over 30 hours, there are five half-lives: 30 ÷ 6 = 5. After each half-life, the count U N C O R R EC TE D Radiation can be harmful mainly because it can cause atoms in other substances to be ionised. The alpha and beta particles have enough mass and/or energy to remove electrons from the outside of atoms, which will change the properties of the atoms. This process also causes the release of reactive particles called free radicals. If this occurs in our bodies, free radicals can go on to damage other molecules that may have particular functions in the body. Damage to DNA in our cells by free radicals can have serious effects on our bodies, because DNA is the molecule that contains instructions for other biochemical Radiation Figure 1.35 Radiation can damage the structure of DNA molecules. Figure 1.36 X-rays use radiation to make images of the bones in the body. processes. DNA can reproduce itself, so the effect of one damaged DNA molecule may be multiplied thousands or even millions of times as copies of the affected DNA are created. Many cancers linked to radiation start this way. When DNA in sperm or egg cells is altered, it may not necessarily affect the individuals themselves but the faulty DNA could be passed on to their children instead. This can result in physical deformities, intellectual disability or genetic diseases in the children of people exposed to dangerous levels of radiation. People who work with radiation take precautions to ensure their cells are not harmed. Lead aprons and protection booths are commonly used by medical practitioners who operate machinery that emit radiation, 32 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 32 28/08/13 2:55 PM such as X-ray and MRI machines. Lead shielding is too dense for the radiation to pass through. Dosimeter badges may be worn if background radiation is likely. These badges contain a photographic film that is developed to measure exposure to radiation. The film darkens in response to radiation – the darker the film, the more radiation exposure. Other versions of dosimeter badges react instantly to radiation, changing colour to indicate the levels of exposure. O FS Figure 1.37 Overexposure to X-rays can be harmful. PA G E Deeper u n d e r s ta n d i n g Figure 1.38 The devastated Chernobyl nuclear power plant. R EC TE D One of the world’s worst nuclear accidents happened at the Ukrainian Chernobyl Nuclear Power Plant in 1986. The accident occurred during a safety check. A surge in power caused the reactor to operate at up to ten times its normal level, which caused a massive explosion and a huge release of radioactive material into the atmosphere. Some workers received fatal doses of radiation in the first few minutes of the accident and it is thought about 50 workers died as a direct result of the accident. Figures for additional deaths linked to cancers caused by increased levels of radiation from the Chernobyl accident are varied, ranging from 5000 to 200 000. PR O Nuclear accidents O R Questions 1.3.2: Types of radiation Remember 1 Identify the three types of radiation. C 2 Define the term ‘half-life’. U N 3 Write the conventional representation of an isotope for each of the following in the form AZX. You may need to use the periodic table to find out the atomic number of the elements. a iodine-131 c technetium-99 b cobalt-60 d fluorine-18 Apply 4 A scientist was using radioactive substances without proper precautions. Explain the possible consequences. 5 An unearthed sample of a ceramic vase was analysed and found to contain 50% carbon-12 and 50% carbon-13. Knowing that the half-life of carbon is 5730, how old is the vase? Explain your answer. 1.3 Radioactivity 33 01_CRA_IS9_77563_TXT_LAY.indd 33 28/08/13 2:55 PM Radiation and medicine L i t e r ac y bu i l d e r O FS used as a treatment for cancer. A small amount of a radioactive substance is placed inside or close to a tumour. The radiation from the substance kills the cancerous cells in the tumour. Many forms of cancer, including skin and breast cancer, can be treated in this way. Brachytherapy is often used in combination with other treatments, including surgery and chemotherapy. Radiation therapy can be used to treat diseases other than cancer, such as in coronary artery disease (a heart disease) to reduce the chance of an artery closing. PR O Radiation is used in medicine in a number of ways. Diagnosis methods that use radiation include X-rays, scans and the injection of radioactive material for nuclear medicine imaging. During radiation therapy, radiation can be used to treat or relieve the symptoms of disease. Radioactive materials can be injected, swallowed or placed directly inside the body, or an external beam of radiation might be used. Radiation therapy inside the body is called brachytherapy. It is most commonly Nuclear medicine technologist: Fran Maestrale E scans. These may be done for a variety of reasons: to diagnose cancer, investigate the extent of arthritis, look for fractures not visible a plain X-ray, or look at bone infection. In other cases, where the blood rather than the bone is of interest, the blood of a patient can be ‘labelled’—mixed with a small amount of radionuclide, a small radioactive nucleus. This can be used to locate the site of an internal bleed. Once the bleed has been located, these patients can go to surgery and the surgeon will know exactly where to begin finding the haemorrhaging vessel so that it may be sealed to prevent further blood loss. A typical day at Maestrale’s work consists of performing a number of these different tests, looking at a variety of different diseases and disorders. Nuclear medicine technologists must be familiar with many organs in the body in order to know whether the images obtained appear normal or abnormal. There is also the opportunity to learn about the various treatments for different conditions patients can have. Although through practice they may be able to interpret images and determine what pathology a person has, nuclear medicine technologists are not qualified to do this. They must present the images to the radiologist, who is responsible for making a diagnosis. Due to the nature of their profession, nuclear medicine U N C R O R Figure 1.39 A technetium99m bisphosphonate bone scan shows up abnormalities within bones. EC TE D PA G Nuclear medicine is a diagnostic imaging method usually located within the X-ray department of hospitals or in private clinics. Most nuclear medicine departments are relatively small, with only a few nuclear medicine technologists (NMTs) at each site. Nuclear medicine is concerned with the function rather than the structure or appearance of organs (as is the case with general X-rays). An NMT performs many procedures each day. Before the first patient arrives at her department, nuclear medicine technologist Fran Maestrale must measure the amount of radioactivity delivered to their department. The isotope, in liquid form, is drawn up into the required amounts and added to ‘cold’ kits, so that the day’s scans can be performed. A cold kit is a vial containing a particular chemical agent that travels to a particular organ once it is introduced to the body. Each test uses a particular compound, which travels to a known organ of the body based on its chemical composition and the way it is introduced into the body. Most people referred to nuclear medicine departments require bone 34 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 34 28/08/13 2:55 PM technologists have a close working relationship with radiologists, surgeons and nurses. Adapted from F. Maestrale, Frankston Hospital, ‘The Job of a Nuclear Medicine Technologist (NMT)’ 1 Outline the training you think a nuclear medicine technologist might need. Why is this? 2 Explain why you think Fran begins her day by measuring the amount of radioactivity delivered to the department. 3 Explain why the nuclear medicine technologist must give the images to a radiologist to make a diagnosis. PR O O FS Figure 1.40 Radioactive dye injected into the blood shows blood flow in the blood vessels. Nuclear technology and carbon dating in Australia PA G E help archaeologists identify the chemicals used to make the pigments, and enable the comparison of elemental composition of the materials to help identify the origins of different artefacts. Ancient exchange networks between different Indigenous groups can be investigated because of this technique. Dr PopelkaFilcoff was awarded a fellowship from the Australian Institute of Nuclear Science and Engineering (AINSE) for this work. AINSE aims to assist cooperation between scientists working in nuclear science and engineering. In an AINSE project carried out in 2010 and 2011, carbon dating techniques were used to analyse cultural material from Mabuiag Island, in the Torres Strait in northern Australia. Previously, materials had been dated at 5000 years old, but recent carbon dating techniques have put the age of the site as 7500 years old – one of the earliest known inhabited sites in N C O R R EC TE D Nuclear science is being used in Australia to uncover some details of both the age and composition of historical materials. A technique called neutron activation analysis has been used at the Australian Nuclear Science and Technology Organisation (ANSTO) to study the composition of ochre, a red pigment used in Indigenous Australian artworks. Using this technique, a sample is placed in a beam of neutrons. The atoms in the sample absorb the extra neutrons and become radioactive. As the nuclei of these atoms decay, they emit gamma radiation. Different elements give off gamma radiation at different energies, so by measuring the energy of the radiation produced, researchers can identify the atoms and hence the elements in the sample. In 2010, Dr Rachel Popelka-Filcoff, a researcher at Flinders University, used this technique to determine the elemental make-up of the ochre pigments used in Aboriginal artefacts. This information will Deeper u n d e r s ta n d i n g overmatter Figure 1.41 Dr Rachel Popelka-Filcoff with some of the Aboriginal artefacts. U Questions 1.3.3: Radiation and medicine Remember 1 Recall two ways radiation can be used in medicine. 2 Identify the name given to internal radiation therapy. Apply 3 Investigate one radioactive isotope used in medicine. State the symbol of the isotope and its uses. Find out how it works, in simple terms. What are the benefits of using this as an alternative to other treatments, such as chemotherapy alone or immediate surgery? What precautions must be taken in handling this radioactive isotope? Does its use have any side effects? overmatter 01_CRA_IS9_77563_TXT_LAY.indd 35 1.3 Radioactivity 35 28/08/13 2:56 PM 3 Recall four radioisotopes. [4 marks] 4 Explain what it means if a substance is radioactive. [2 marks] 5 Describe how and where radionuclides for industry and medicine are produced. [3 marks] Making connections 13Apply your understanding of the structure of atoms and radioactivity to outline reasons for the following differences in properties: PA G Apply 12Radiation produced from the decay of radioactive atoms can be very dangerous, but it can also be used in life-saving medical treatments. Imagine you are a doctor wanting to reassure a patient about a treatment based on exposure to radiation. List the main ideas you would tell your patient. [2 marks] FS 2 Describe a beta particle. Write its symbol, including the atomic number and mass number in the correct positions. [2 marks] Critical and creative thinking O 1 Describe an alpha particle. Write its symbol, including the atomic number and mass number in the correct positions. [2 marks] 11Describe why the Bohr model gives us much more information than Dalton’s early model of the atom. [2 marks] PR O Checkpoint Remember and understand E 1.3 Radioactivity EC TE D 6 Draw a radioactive decay curve for a substance that starts with an activity of 1600 counts per minute and has a halflife of 2 hours. [5 marks] 7 Calculate the half-life of a radioactive substance that decays from 400 counts per minute to 50 counts per minute in 9 hours. [3 marks] R Analyse and evaluate O R 8 Alpha radiation consists of particles that can be easily stopped by our skin. Explain why they are still considered a potential risk to our health. [2 marks] U N C 9 Consider the composition of alpha and beta particles. When a beam of alpha particles and a beam of beta particles are fired into an electric field, they move in opposite directions. Explain why this is. [2 marks] a beta particles can pass through human skin but alpha particles cannot [2 marks] b beta particles can be stopped by a few millimetres of lead foil, but gamma rays will pass through several centimetres of lead [2 marks] c carbon-14 is a radioactive isotope but carbon-12 is stable [1 mark] d alpha decay of an atom always produces a lighter atomic nuclei, but beta decay results in a nuclei that may be of similar mass to the original atom [2 marks] e uranium-238 has a half-life that is 1000 times longer than uranium-234 [2 marks] 10Outline how the work of Marie Curie influenced future work in different areas of science. [2 marks] TOTAL MARKS [ /40] 36 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 36 28/08/13 2:56 PM 1 Fill in the gaps using the words in the Word Bank below: All ____________ is made up of atoms. Atoms are composed of positively charged ____________ and ____________ neutrons in the nucleus of the atom, surrounded by shells of orbiting negatively charged ____________. Our knowledge and understanding of the structure and properties of ____________ has changed as more ____________ has been discovered. The atomic ____________ has been developed by Democritus, Dalton, Thomson, Rutherford, Chadwick and many others through series of ____________ and research. 1 Model Atoms Evidence Matter Neutral Beta Experiments Identify that all matter is made up of atoms 2 Use a periodic table to help you to complete the table of information about some elements. [3 marks] Symbol Relative atomic mass Ca Neon Atomic number Protons Unstable Chapter review 6 Identify one piece of observational evidence that supports the model of electrons in stable orbits called shells around the nucleus. [1 mark] 7 A student draws a picture of an atom showing a solid nucleus with protons and neutrons inside it and electrons orbiting around it. Outline why this is not an accurate drawing. [3 marks] EC TE D Element name O Gamma PR O Electrons E Alpha PA G Word bank FS When the nuclei of an atom is ____________, it can decay and release energy and particles as radiation to become more stable. ____________ particles are made up of two protons and two neutrons. ____________ particles are electrons. ____________ radiation is the release of high-frequency electromagnetic energy. 20 C O R R 3 A quick look at a periodic table of elements shows all atoms have an atomic number that is a whole number, but most have a mass number that is not a whole number. Explain why this is so. [2 marks] U N Outline the developments to the atomic theories and models as a process of refinement and review of the scientific community 4 Describe the main features of John Dalton’s idea about atoms. [2 marks] 5 Identify the most important piece of evidence Rutherford had that supported his model of the atom. [1 mark] 8 The model of the atom has changed over time as new information was discovered and evidence was established that either extended or refuted the model. For the Rutherford model and the Bohr model of the atom, identify what supporting evidence led to the establishment of the models. [2 marks] Describe the structure of atoms in terms of protons, neutrons and electrons 9 Look at the periodic table in Figure 1.19 and use the information to identify two elements other than oxygen and carbon that have one predominant isotope. Explain how you decided these elements probably had one main isotope. [3 marks] 2 Radioactivity chapter review 37 01_CRA_IS9_77563_TXT_LAY.indd 37 28/08/13 2:56 PM Identify that radioactivity arises from the decay of nuclei in the form of particles and energy 11Explain why radiation emitted as an alpha particle is much more dangerous than beta particles. [2 marks] FS 14A medical scientist has a choice of isotopes to use for a procedure. One has a half-life of 3 hours and the other a half-life of 62 minutes. If you were the patient, which isotope would you prefer? Explain your answer. [2 marks] 15Evaluate the advantages and disadvantages of using nuclear power to generate electricity. [2 marks] PA G U N C O R R EC TE D TOTAL MARKS [ /30] 12A specific atom of a particular radioactive isotope has not decayed in 20 half-lives of time. Explain how this is possible. [2 marks] 13A radioactive isotope used in medicine has a half-life of 23 minutes. The isotope is produced in a nuclear reactor and then couriered to hospitals where it is used with patients. If it arrives at the hospital more than 3 hours after production, it is said to be useless. Explain why that would be. [3 marks] O 10An atom of calcium, if placed into water, will react to form an ion of calcium (Ca2+) in solution. During that reaction with water, hydroxide ions (OH–) and hydrogen gas will be formed from the breakdown of water molecules. Predict where the electrons lost from the calcium are transferred during this reaction. [2 marks] Evaluate the benefits and limitations of the medical and industrial use of nuclear energy PR O Use models to describe the arrangement of subatomic particles in common elements (additional) E 1 REVIEW CHAPTER Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 38 28/08/13 2:56 PM radioactivity. What did he contribute to this important work? What scientific units are named after him? Did his children, like those of Marie and Pierre Curie, also follow careers in science? Targeted alpha therapy CERN Henri Becquerel shared a Nobel Prize with Marie Curie for work in discovering 1 What new science laboratory skills have you learned in this chapter? 6 What are the risks of using radioactive materials? 3 What were the most difficult aspects of this topic, and why? My future U N C O R R 4 Why is it important to know about atoms and radioactivity? Key words alpha particle atomic number atomic theory atoms background radiation beta particle Bohr model Brownian motion carbon dating compounds decay electron electron shell electron configuration elements emission spectrum exponential decay curve free radicals gamma ray half-life ion ionisation isotope law of constant composition law of simple multiple proportion mass number neutron nucleus periodic table proton radiation radioactive radioactive decay radionuclide relative atomic mass Rutherford nuclear model shell diagram Thomson plum pudding model valence shell FS 2 What was the most surprising thing that you found out about atoms? My world 1 REVIEW CHAPTER O 5 What changes to our lives have occurred as we have increased our understanding of atoms? EC TE D Reflect Me PR O Henri Becquerel The European Organization for Nuclear Research, known as CERN, is based on the border of France and Switzerland. It has been responsible for developing scientists’ understanding of atoms. What countries collaborate in this project? What types of scientist work at CERN? What current work is occurring at CERN? What is the Large Hadron Collider? E Targeted alpha therapy (TAT) is a new therapy for the control of some cancers. How does this form of therapy work? What types of cancer are treated by this method? How widespread is its use? Identify the risks associated with this form of radiotherapy and how are they reduced. PA G Research Choose one of the following topics for a research project. Some questions have been included to help begin your research. Present your report in a format of your own choosing. 7 Do you think it is important to find out more about the inside of atoms? 8 What future uses of radioactive materials do you think will happen in your lifetime? 2 Radioactivity 1 chapter chapter review 39 01_CRA_IS9_77563_TXT_LAY.indd 39 28/08/13 2:56 PM O FS in nuclear reactions, which is why stars give out so much energy. All elements on the Earth were formed in stars that were far bigger than our Sun. Scientists can only produce new elements in nuclear reactors and similar devices. In 2008, the Large Hadron Collider was turned on for the first time. This gigantic particle accelerator, a circular tunnel 100 m underground and almost 27 km long, was set up to study the fundamental building blocks from which atoms are made. Physicists hope Figure 1.42 Matter is sent in all directions when a star explodes. U N C O R R EC TE D PA G E MA K ING C O NNE C TI O NS From the evidence collected so far, scientists think the Universe began as an exceptionally dense cluster of fundamental particles from which all matter is made. Suddenly this mass ‘exploded’, sending matter in all directions. This matter collected together in various places, forming the stars and eventually planets. This theory about how the Universe began is known as the Big Bang theory. Scientists also think the different elements were formed in the stars as bigger and bigger nuclei built up from this matter, in reactions known as nuclear reactions. This theory is known as the nuclear genesis theory. Vast amounts of energy are released PR O 1 Atoms: past, present and future 40 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4 01_CRA_IS9_77563_TXT_LAY.indd 40 28/08/13 2:56 PM PR O O 1 Recall where scientists think the elements that occur naturally on the Earth were formed. R O R C N U Figure 1.45 Carbon nanotubes are extremely thin (diameter 10 000 times smaller than a human hair), hollow cylinders made of carbon atoms. They can be used as ‘packets’ to deliver substances to specific sites in the body. FS the impact of exposure to radiation. Much of the recent research into this area has been aimed at improved targeting of the damaged cancer cells that need to be killed, so that overall doses of the radiation required can be reduced. One strategy is the use of carbon nanotubes to contain the radioactive material. These nanotubes could then be delivered (using selective chemical ‘tags’ attached to the nanotube) to very specific sites in the body. 2 Describe what this implies about all the atoms now present on the Earth. Would they always have been here? PA G E 3 Explain why you think there are only around 100 naturally occurring elements. Why aren’t there many more? 4 Explain why the production of nuclear wastes with long half-lives is a potential environmental problem. EC TE D to ‘see’ what happened during the Big Bang. One way to classify the elements we encounter on the Earth is according to whether they occur naturally on Earth or if they have only been produced in nuclear reactors and particle accelerators. A further way to classify elements is to show which ones have radioactive isotopes. Only atoms with unstable nuclei are radioactive. Many change from one nucleus to another, and then to another, until they eventually form a stable nucleus. Some nuclei are so unstable that they only last a fraction of a second, which makes them very difficult to detect. This is the kind of challenge facing scientists who are trying to discover and gather evidence for the existence of elements with atomic numbers greater than 118. Nuclear power and radiotherapies are areas of science with unknown futures. Some people see the increase in the use of nuclear power as a way of reducing climate change because the amount of carbonbased fuels being burned would be reduced. Others consider this a dangerous move as a large amount of radioactive waste would be produced from the process, with halflives of thousands or even millions of years. Australia currently does not produce any of its domestic energy using nuclear power stations, whereas nuclear power contributes significantly to electricity generation in other countries. Recent accidents, such as damage to the Japanese nuclear power station in 2011 as a result of an earthquake and tsunami, have raised further questions about the safety of nuclear energy. However, Australia is rich in deposits of uranium and the sale of these resources is of major benefit to the Australian economy. Sales of nuclear fuels remain controversial, because the same materials can also be made suitable for nuclear weapons. In the area of medicine, it is expected that as our understanding of atoms and radioactivity increases, more effective treatments will emerge and will save many more lives and improve quality of life. But even in medicine there are concerns about 5 Research which countries rely on nuclear energy to produce a large proportion of their electricity. Why do you think the governments of these countries have chosen to do this? Figure 1.43 The Large Hadron Collider. 6 Research the countries to which Australia currently exports uranium. 7 Why do you think there are still concerns about the safety of radiotherapies? 8 Outline what a carbon nanotube is. 9 Do you find it surprising that, even after years of research, scientists are still trying to find out more about the atom? Discuss you reasoning. Figure 1.44 Scientists at the Fukushima nuclear power station after the earthquake and tsunami in Japan in 2011. 1 making connections 41 01_CRA_IS9_77563_TXT_LAY.indd 41 28/08/13 2:56 PM