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1.10 Atomic structure What is an atom made of? � Key terms Element – A substance which contains just one type of atom. An image showing atoms on the surface of a metal, produced by a method called scanning tunnelling microscopy. About 200 years ago, when scientists first realised that all matter was made up of atoms, they thought that atoms were the smallest particles. But now we know that atoms are made up of even simpler sub-atomic particles – protons, neutrons and electrons. D Atomic number – The number of protons in the nucleus of an atom. All matter is made up of particles called atoms. Atoms are very, very small. It is not possible to see an atom even under the most powerful light microscope. However, scientists and engineers have developed ways of producing images of atoms using other methods. ft All matter in the Universe is made up of atoms. But how much do we know about atoms? Discuss with a partner whether these statements are true or false: • Atoms cannot be split up into anything smaller. • Atoms are solid objects and nothing can pass through them. • There are millions of different kinds of atoms. • No-one has ever seen an atom. ra Get started Mass number – The total number of protons and neutrons in the nucleus of an atom. At the centre of an atom is a positive nucleus. If an atom were the size of The O2 arena, the nucleus would be the size of a ping-pong ball. The nucleus is surrounded by electrons, arranged into layers or shells. Sometimes these electron shells are called energy levels. nucleus, containing protons and neutrons Did you know? Because the nucleus is so small, most of the atom is actually empty space. When scientists tried firing sub-atomic particles at a thin gold film, they found that most of them passed straight through. Only a tiny number of the sub-atomic particles hit the nuclei of the gold atoms and bounced back. 32 p n n p n electron shells, containing electrons p n Key n neutron p proton electron An atom of lithium contains a nucleus surrounded by electron shells (energy levels). BTEC First Principles of Applied Science Principles of Science UNIT 1 The sub-atomic particles Each of the sub-atomic particles has different properties, as shown in this table. Particle Relative charge Relative mass Where in the atom is it found? Proton +1 1 In the nucleus Neutron 0 1 In the nucleus Electron −1 Almost 0 (actually 0.0005) In shells surrounding the nucleus Link Lesson 1.12 discusses the arrangement of elements in the periodic table. Atomic number and mass number � So far scientists have identified 118 different types of atom. This means there are 118 different elements. mass number 7 Li ft Each element has a fixed number of protons in the nucleus. For example, all atoms of lithium have 3 protons in the nucleus. This is the atomic number for lithium. There are no other elements with an atomic number of 3. Activity A 3 ra atomic number The nuclear symbol for the lithium atom shows you the atomic number and mass number of the atom. D The mass number of an atom is the total number of protons and neutrons in the nucleus. Lithium has 4 neutrons, so its mass numbner is 7. The nuclear symbol for an atom is its chemical symbol with the atomic number and mass number. Worked example You can use the nuclear symbol for an atom to work out the number of protons and neutrons in the nucleus. If a phosphorus atom has a nuclear symbol 31 15P, how many protons and neutrons are there in its nucleus? 1 Using the nuclear symbols shown for these atoms, write down the numbers of protons and neutrons in the nucleus of each atom. 16 8 O 23 11 Na 2 An atom of potassium (symbol K) has 19 protons and 20 neutrons. Write down the nuclear symbol for this atom. Take it further Neutrons have an important role to play in the nucleus of an atom. They prevent the positive protons from repelling each other and blowing the nucleus apart. Step 1 The atomic number is 15. This is the number of protons in the nucleus. Step 2 The mass number is 31. This is the total number of protons and neutrons in the nucleus. Step 3 So, the number of neutrons 5 31 2 15 5 16. Just checking 1 Copy and complete this sentence about the structure of atoms. Atoms contain a nucleus with a ___________ charge, surrounded by ___________ in shells. The size of the nucleus is much __________ than the overall size of the atom. Lesson outcomes You should know about: • the structure of an atom • the relative size of the nucleus • that atoms of a given element have the same number of protons in the nucleus • the meaning of the terms ‘atomic number’ and ‘mass number’. 33 1.11 Isotopes and relative atomic mass Key terms Two kinds of carbon atoms? � Isotopes – Atoms of the same element which have the same number of protons but different numbers of neutrons. Almost all of the carbon atoms found in living things and in the rocks, seas and atmosphere of our planet have the nuclear symbol 12 6 C. This is called carbon-12. It is so stable that the nuclei of these carbon atoms have not changed since the Solar System was formed over 4 billion years ago. Relative atomic mass (of an element) – The mass of an atom of that element compared to a standard mass. A tiny fraction of the carbon atoms on Earth have the nuclear symbol 14 6 C. This is known as carbon-14. The nuclei of carbon-14 atoms have two extra neutrons when compared with carbon-12, so the two atoms have different mass numbers. The two different carbon atoms are called isotopes of the element carbon. There is also a third isotope of carbon with a mass number of 13. It has the symbol 13 6 C. Did you know? Lesson 1.10 explains how to work out the number of protons and neutrons from the nuclear symbol. Carbon-14 is radioactive. This means that when a living organism dies, the carbon-14 atoms it contains gradually decay into more stable atoms. Scientists can use this fact to find the age of the remains of long-dead organisms. This is called carbon dating. D Write down the number of protons and neutrons in the following atoms: • carbon-12 • carbon-14 Ötzi the ‘Iceman’ was found in a glacier in the Alps. Archaeologists used the amount of carbon-14 remaining in his body to calculate that he died about 5000 years ago. ra Activity A ft Link Take it further Relative atomic mass is a way of comparing the mass of an atom to a standard mass. In the past, all masses were compared to a hydrogen atom – but now they are all compared to 1/12 of the mass of a carbon-12 atom. 34 Relative atomic mass � Only a tiny fraction of carbon atoms have a mass number of 13, or of 14. All the rest have a mass number of 12. So, the average mass of a carbon atom is 12.01. This is called the relative atomic mass of carbon. BTEC First Principles of Applied Science UNIT 1 Principles of Science Isotopes and relative atomic mass � The most common isotope of chlorine is 35 17 Cl. But about a quarter of all chlorine atoms 37 are a different isotope: 17 Cl. Because this is quite a high proportion, the relative atomic mass is not a whole number – it actually works out to be 35.5. Cl 37 Cl 35 Cl 35 35 Cl 35 35 Cl 37 Cl 35 Cl Cl Exam tip You may be required to use data about the abundance of isotopes to predict which elements will have relative atomic masses that are not whole numbers. You also need to be able to calculate the relative atomic mass from these data. About three-quarters of all chlorine atoms are chlorine-35 isotopes. The rest are chlorine-37. There are many other elements like chlorine, where the relative atomic mass is not a whole number because of the presence of different isotopes. The table shows three examples. Relative atomic mass Lithium 6.9 Magnesium 24.3 Copper 63.5 Isotopes present and percentage abundance ft Element 6 3 Li 7 3 Li (7.6%) (92.4%) (79%) (10%) (11%) ra 24 12 Mg 25 12 Mg 26 12 Mg (70%) (30%) D 63 29 Cu 65 29 Cu Worked example Activity B 11 Boron has two isotopes: 10 5 B (20%) and 5 B (80%). What is the relative atomic mass of boron? Step 1 First, multiply the mass number of each isotope by its percentage abundance. Step 2 Then, add all these values together. Step 3 Finally, divide by 100 to get the average. The element thallium has two isotopes: 203 81 TI 205 (30%) 81 TI (70%) Use this information to calculate the relative atomic mass of thallium. So, for boron, the relative atomic mass 5 (10 3 20) 1 (11 3 80) 5 1080 5 10.8 100 100 Just checking 1 Look at these nuclear symbols for four atoms, A–D: 14 16 15 14 7 A 8 B 7 C 6 D (a) How many of these atoms have eight neutrons in their nucleus? (b) Which two of these atoms are isotopes of each other? Explain your answer. 2 Bromine has two isotopes: 50% of bromine atoms have a mass number of 81 and 50% of bromine atoms have a mass number of 79. What is the relative atomic mass of bromine? Explain your answer. Lesson outcomes You should know about: • relative atomic mass and isotopes • how to calculate relative atomic mass from percentage abundances. 35