Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Lesson 4: Atomic Structure OBJECTIVES Define the word atom Describe the four points of Dalton's atomic theory of matter Identify and describe the two kinds of electrical charges Describe how particles with the same charge affect each other Describe how particles of different charges affect each other Discuss how atoms are related to electricity Explain how cathode rays and radioactivity are related to atomic structure Explain how Rutherford's experiment showed the existence of the nucleus Describe alpha, beta, and gamma radiation Name and describe the three subatomic particles Determine the number of protons, neutrons, and electrons in an atom or ion Explain how an ion differs from an atom Determine number of subatomic particles from the symbol of a nuclide Identify isotopes Calculate average atomic mass from relative abundance Describe the changes that accompany nuclear reactions Define radioactivity Perform calculations with half-lives Atomic Structure, Atomic Number, Mass Number The atom is the basic unit of matter. It is the smallest particle of an element that still has the characteristics of that element. Every atom has a positively charged central nucleus, composed of positively charged protons (1+) and uncharged neutrons. The nucleus is surrounded by a number of negatively charged electrons (1-). Like magnets, particles of opposite charge are attracted to one another, and particles with the same charge are repelled from one another. The structure of the atom that we are familiar with today has a long history to which many people contributed. The discovery of these particles and their arrangement, relative masses, and charges required many experiments. The word atom comes from the Greek word atomos, meaning "indivisible," and indeed the idea that all matter is composed of tiny, indivisible particles goes back to ancient Greece. The first scientist in modern times to revive this idea was a British scientist named John Dalton. In 1803, after observing that elements in a given compound always exist in the same proportions, Dalton formulated his atomic theory of matter. The theory had four main points: Each element is composed of extremely small particles called atoms All atoms of a given element are identical; the atoms of different elements are different and have different properties (including different masses) Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created nor destroyed in chemical reactions Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms. Michael Faraday was an English chemist who, in 1832-33, carried out a series of experiments attempting to use electricity to isolate elements from known compounds. His work led him to discover that the amount of electricity applied to a sample compound is related to the amount of an element that is isolated. He concluded that the structure of the atom must play a role in this. Building on Faraday's experiments, English physicist J. J. Thompson discovered that electricity consisted of tiny subatomic particles called electrons. In 1897, Thompson was the first person to measure the mass-to-charge ratio of a single electron. He found that the particles he observed were common to all atoms. Thomson also knew that the negative charge he measured must be balanced by a particle of positive charge, since there was no overall charge on an atom. Thomson devised his own model of the atom, which is often described as the blueberry muffin model: he pictured an atom as a uniform ball of positive charge, with the electrons scattered uniformly throughout, like blueberries in a muffin. The American physicist R. A. Millikan determined the unit of charge using oil drops and an electrical field. By calculating the velocity with which a charged drop was deflected from the electrical field, Millikan was able to calculate the charge on the drop. The mass could also be calculated. By performing many experiments, Millikan was able to calculate the charge on an individual electron and the mass of an individual electron. The true arrangement of particles in the atom was discovered by Ernest Rutherford, another English physicist. Rutherford performed his famous platinum and gold foil experiments with a college student who was named Ernest Marsden. They determined that some positively charged helium nuclei, alpha particles (He2+) were deflected by platinum foil, at angles greater than 90 degrees. Sometimes the particles were deflected almost 180 degrees, back the way they came! For an alpha particle to be deflected that much, it must pass very close to a group of particles that also had a positive charge. The group of particles would have to have a strong positive charge, concentrated in a relatively small area. That is what the nucleus of an atom is – a relatively small area with a concentrated positive charge. The individual protons and neutrons were more difficult to isolate and identify. At this time, it was known through Rutherford’s and others experiments that while the proton and electron had the same charge, that the mass of the proton was much greater. In any atom, the number of protons and electrons are the same. The existence of an uncharged particle was discovered by comparing the number of positive charges (number of protons) in an atom with the mass. The number of mass units was almost twice the number of protons. Where was the extra mass coming from? James Chadwick’s experiments with beryllium and positively charged alpha particles revealed the presence of the neutron. By bombarding beryllium with alpha particles, Chadwick was able to observe that an uncharged particle was given off, which had the same mass as a proton. Chadwick used a specific type of beryllium, beryllium-9. The number 9 is the mass number, which indicates the total number of protons and neutrons in an atom. The number of protons in an atom of a specific element always identifies the element. The number of neutrons in atoms of the same element may be different. When neutrons are different in atoms of the same element, they are called isotopes. The element neon has two common isotopes, neon-20 and neon-21. Both isotopes have 10 protons, since neon is element number 10 in the Periodic Table. By subtracting the number of protons from the mass number, we see that neon-20 has 10 neutrons, and neon-21 has 11 neutrons. Beryllium-9 has 4 protons and 3 neutrons. Mass number = number of protons + number of neutrons. Each element is identified by a unique atomic number, which is shown on the Periodic Table. The atomic number is equal to the number of protons in each atom of the element. The number of protons and electrons in a neutral atom of an element is always the same. How does this work? Let’s complete the chart for an example, oxygen-16. The mass number is the number after the dash, 16. Symbol Atomic Number O 8 Number of Protons 8 Number of Electrons 8 Mass Number 16 Number of Neutrons 8 For an atom of Ca-40, the mass number is the number after the dash, 40. The atomic number can be obtained by looking for Ca on the Periodic Table. The atomic number is 20, the number of protons. Mass number – number of protons = number of neutrons. Number of neutrons equals 20. If no charge is specified, the number of electrons is the same as the number of protons. The number of electrons is therefore 20. Symbol Atomic Number Ca 20 Number of Protons 20 Number of Electrons 20 Mass Number 40 Number of Neutrons 20 We can follow this process for any isotope for which a mass number is designated. If we do not know the mass number, we can use the Periodic Table to obtain an average atomic mass, which is usually a decimal, then round it to the nearest whole number. For instance, the element sulfur, S, has an average atomic mass on the Periodic Table of 32.07. By rounding this number to the nearest whole number, 32, we now have the mass number for a common isotope of sulfur. We can obtain the number of protons from the atomic number on the Periodic Table, and the number of electrons will be the same if no charge is indicated. Ions and Ion Formation The number of electrons can vary if charged particles called ions are formed. How can we determine the number of protons and electrons in an ion? Remember that the atomic number always gives the number of protons. We can find the atomic number on the Periodic Table, by looking in the same box as the symbol for an element. Each proton has a 1+ charge. An atom of an element has the same number of protons and electrons, and that means the total charge adds to zero. Atoms of an element are often said to be neutral for this reason. Example: (Use the formula: positive charge + negative charge = net charge) The element sodium, Na, has atomic number 11, meaning 11 protons and 11 electrons: (11+) + (11-) = 0. There is zero net (overall) charge, the atom is neutral; it is not an ion. An atom of chlorine has atomic number 17, 17 protons and 17 electrons: (17+) + (17-) = 0. Zero net charge, this is a neutral atom. Let’s try this for ions. An ion is an atom that carries a charge either through gaining or losing an electron. A cation is an atom that loses an electron and has a positive charge. An anion is an atom that gains an electron and has a negative charge. We do not observe negative ions by themselves, they are balanced by positive ions. Using an everyday substance as our example, we can determine the number of protons and electrons for a positive ion, Na1+, and a negative ion, Cl1-. These ions are found in table salt, sodium chloride, NaCl. The ions are formed from the elements sodium, Na, and chlorine, Cl. The sodium ion, Na1+, has atomic number 11. This means 11 protons, 11 positive charges. The total charge is 1+, overall. That means that there is one “extra” or unbalanced charge. What about the other 10 positive charges? They are still there in the nucleus, and their charge is balanced by 10 electrons. Use the formula positive charge + negative charge = net charge. We already know that 11 protons = 11+, and net charge = 1+ ; so substitute into the equation: (11+) + number of negative charges = 1+ The math on this one is easy! There are 10 negative charges (10-), which means 10 electrons. The sodium ion, Na1+ , has 10 electrons; a neutral atom of sodium has 11 electrons. The sodium atom loses one electron to form the positive ion. Where does the electron go? Let’s take a look at the negative ion in the NaCl compound. For the chloride ion, Cl1-, the process is the same. The atomic number is 17, indicating 17 protons. The net charge is 1-. How many of those negative charges are balanced with a positive charge? Remember: Positive charge + negative charge = net charge (17+) + number of negative charges = 1-number of negative charges = 18- , and number of negative charges equals number of electrons, 18! A neutral chlorine atom has 17 electrons. The chloride ion has 18 electrons. A chlorine atom has to gain an electron to form the negative ion. Therefore, ions are formed by the loss or gain of electrons. This process works for any ion, for which you know the symbol and charge. The symbol allows you to determine the atomic number by looking at a Periodic Table. Calculating the Average Atomic Mass The average atomic mass of an of its isotopes. Mass Element Isotope Number 12 Carbon C 12 6 13 C 13 6 element is determined by averaging the natural abundance Mass (amu) Fractional Abundance 12 (exactly) 98.89% Average Atomic Mass 12.01 13.003 1.11 Chloride Silicon 35 Cl 17 37 Cl 17 28 Si 14 29 Si 14 30 Si 14 35 34.969 75.53 37 36.966 24.47 28 27.977 92.21 29 28.976 4.70 30 29.974 3.09 35.45 28.09 Example: Use the table above to calculate the average atomic mass of silicon. Multiply the mass by the percent abundance for each of the three isotopes of silicon. (Change the percentage abundance to a decimal by moving the decimal point two places.) (27.977) (0.9221) = 25.80 amu (28.976) (0.0470) = 1.362 amu (29.974) (0.0309) = 0.926 amu Find the sum of the results. 25.80 + 1.36 + 0.926 = 28.08 amu Round to three significant figures, 28.1 amu. The average atomic mass of silicon is 28.1 amu. Nuclear processes and Radioactivity The French researcher Antoine Henri Becquerel, made the first observations of natural, spontaneous radioactivity. While studying fluorescent properties of substances, he noticed that photographic plates placed next to certain uranium compounds became darkened, as if they had been exposed to light! This is not what he had intended to study, but this accidental discovery had an enormous impact. One of the students in his lab was Marie Curie, who went on to win the Nobel Prize in physics in 1903 with her husband Pierre Curie. She was awarded another Nobel Prize in 1911 for chemistry for her work on radium and polonium. Marie Curie is one of only three people to win two Nobel Prizes in science. Her daughter and son-in-law shared a Nobel Prize in chemistry in 1935. Because of these discoveries, Rutherford and other researchers were aware of the existence of elements that are naturally radioactive. This means that the elements undergo a specific type of change by emitting particles of different sizes and charges, with differing amounts of energy, from the nuclei of atoms. Nuclear decay (radioactive decay) causes the number of protons in the nucleus to change. Since the identity of an element is defined by the number of protons in its nuclei, nuclear decay results in the transmutation of one element to another. This type of change, nuclear change, is very different from ordinary chemical reactions. Two types of particles are emitted during natural nuclear decay: alpha particles and beta particles. Alpha particles consist of two neutrons and two protons bound together--the equivalent of a positively charged helium ion (He2+). Let’s ignore the charge for a moment, and look at the symbol in a new way. Alpha particles are often written in the following form: 4 He 2 The 4 in the upper left is the mass number. The 2 in the lower left is the atomic number. The helium ion still has its 2+ charge; it is just not shown in many nuclear reactions. The arrangement of particles in the nucleus and the changes that occur in radioactive decay processes get special attention, and the charge is typically ignored. Charges are most often important in ordinary chemical reactions, where they receive much more consideration. An example of alpha decay is the naturally occurring decay of uranium-238 238 U 92 234 4 Th + 90 He 2 An alpha particle also has a 2+ charge, even though we do not see/write the charge in nuclear reactions. Note that the element starting material on the left of the arrow, uranium238, is not the same as the element products helium-4 or thorium-234. A beta particle is formed when a neutron from the nucleus breaks down into a proton and an electron. The electron, called a beta particle, is expelled from the nucleus. It is important to remember that a beta particle (β-) is not one of the original set of electrons that are outside the nucleus. An example of naturally occurring beta decay is thorium-234: 234 Th 234 Pa 90 β- + 91 The beta symbol is used to be sure that we realize that this is not an electron from outside the nucleus. We can also recognize the correct mass and charge by using another symbol, e: 234 Th 90 234 Pa 91 0 + e -1 Note that the element starting material on the left of the arrow, thorium-234, is not the same as the element product protactinium-234. In addition to alpha and beta particles, nuclear decay also produces gamma rays. Gamma rays are very high energy electromagnetic radiation with essentially no charge or mass. They have more penetrating power than X-rays. Half-Lives Not all radioactive elements decay at the same rate. Some artificially created isotopes are very unstable and only exist for a few minutes before breaking down. At the other extreme, the radioactive isotope rubidium-87 decays to strontium-87 extraordinarily slowly; it takes 60 billion years (!) for 50% of a particular sample of 87Rb to decay. The time it takes for one half of a sample of a radioactive isotope to decay is called the isotope's half-life. The halflife of 87Rb is therefore 60 billion years. Carbon-14 has a half-life of 5,730 years, and the half-life of the artificial radioisotope iodine-131 is 0.022 years (about eight days). Part 1 Discovering Atomic Structure: Select the correct answer to each of the following questions. (Each question is worth one point) 1. The scientist who suggested that the structure of the atom was somehow related to electricity was Ben Franklin Democritus Michael Faraday John Dalton 2. The physicist Henri Becquerel discovered radioactivity while working with a sample of radium silicon curium uranium 3. Which of the following is not a component of the radiation emitted by a radioactive sample? alpha radiation delta radiation gamma radiation beta radiation 4. Rutherford's alpha-scattering experiment showed that the charge on the nucleus of the atom must be positive neutral negative too small to be detected Part 2 Select the term from the list that matches each description below. (Each question is worth one point) 5. __________ spontaneous emission of radiation from an element. radioactivity static electricity electron 6. __________ negatively charged particle found outside the atomic nucleus. static electricity electron coulomb 7. __________ small core at the center of an atom containing a positive charge. alpha particle gamma radiation atomic nucleus 8. __________ particle with a 2+ charge that is emitted by radioactive elements. electron cathode ray alpha particle Part 3 Answer the following question. (Each question is worth one point) 9. Through a series of experiments, what was Millikan able to determine about electrons? By performing many experiments, Millikan was able to calculate the charge on an individual electron and the mass of an individual electron. Part 4 Modern Atomic Theory: Use the periodic table to determine how many protons, neutrons, and electrons are present in each of the following atoms. Type your answers in the text box provided. Click here to view the periodic table (Each question is worth nine points) 10. Atom Protons Neutrons Electrons iodine-125 53 72 53 potassium-39 19 20 19 iron-56 26 30 26 Atom Protons Neutrons Electrons iodine-125 53 72 53 potassium-39 19 20 19 iron-56 26 30 26 11. Atom Protons Neutrons Electrons oxygen-18 8 10 8 cobalt-60 27 33 27 hydrogen-3 1 2 1 Atom Protons Neutrons Electrons oxygen-18 8 10 8 cobalt-60 27 33 27 hydrogen-3 1 2 1 Part 5 Directions: Type the chemical symbol for each of the ions described below. Since we can't use special formatting tools in our textboxes, we'll have to use another way of writing these symbols. Look at the following example. Type the chemical symbol for the ion with 33 protons and 36 electrons. The answer is: arsenic with a charge of 3- ; you would type: As 3- (Each question is worth one point) 12. Cl- has 17 protons and 18 electrons. Cl13. Li+_ has 3 protons and 2 electrons. Li+ 14. Mg^2+ has 12 protons and 10 electrons. Mg^2+ 15. O^2- has 8 protons and 10 electrons. O^2Part 6 Use the periodic table to determine the number of protons and electrons in each of the following ions. Type your answers in the text box provided. Click here to view the periodic table (Each question is worth eight points) 16. Ion Protons Electrons 2+ Cu 29 27 F9 10 + H 1 0 Na+ 11 10 Ion Protons Electrons Cu2+ 29 27 F- 9 10 H+ 1 0 Na+ 11 10 Part 7 Atomic Structure: Choose the best answer for the following questions. (Each question is worth one point) 17. The nucleus of the atom is composed of which two particles? Proton and electron Proton and neutron Electron and neutron 18. The charge of a proton is: +1 no charge -1 19. The charge of a neutron is: +1 no charge -1 20. Experiments by J.J. Thomson led to the discovery of the: Proton Neutron Electron 21. The nucleus of an atom is a: Relatively small area with a concentrated positive charge Relatively small area with a concentrated negative charge Relatively large area with a weak positive charge Part 8 Mass Number: Choose the best answer for the following questions. (Each question is worth one point) 22. The number of ______ in an atom of a specific element is always the same as the element's atomic number. Protons Electrons Neutrons 23. The ________ indicates the total number of protons and neutrons in an atom. Element number Mass number Isotope number 24. When the number of ________ is different between atoms of the same element, these atoms are called isotopes. Protons Neutrons Electrons 25. In a neutral atom, the number of ________ and ________ is the same. Protons, neutrons Neutrons, electrons Protons, electrons 26. If you’re not given the __________ in a problem, you can determine it by rounding the average atomic mass of the element given on the periodic table to the nearest whole number. Mass number Element’s symbol Atomic number Part 9 Ions and Ion Formation: Choose the best answer for the following questions. (Each question is worth one point) 27. How many protons and electrons are present in a nitrogen atom? 8 protons and 8 electrons 7 protons and 7 electrons 6 protons and 7 electrons 28. How many protons and electrons are present in an argon atom? 18 protons and 18 electrons 18 protons and 17 electrons 18 protons and 16 electrons 29. How many protons and electrons are present in a potassium atom? 18 protons and 19 electrons 20 protons and 19 electrons 19 protons and 19 electrons 30. How many protons and electrons are present in a nitrogen atom? 6 protons and 6 electrons 7 protons and 7 electrons 7 protons and 6 electrons 31. What is the name of the element that has atoms that contain 5 protons? Boron Chlorine Potassium 32. What is the name of the element that has atoms that contain 11 protons? Magnesium Sodium Hydrogen 33. What is the name of the element that has atoms that contain 25 protons? Cobalt Oxygen Manganese 34. What is the chemical symbol for the ion with 12 protons and 10 electrons? Mg Mg2+ Ne235. What is the chemical symbol for the ion with 95 protons and 89 electrons? Am6+ Ac6- Am636. What is the chemical symbol for the ion with 28 protons and 30 electrons? O Zn2+ Ni237. What is the chemical symbol for the ion with 10 protons and 14 electrons? Ne4- Ne4+ Si 38. What is the chemical symbol for the ion with 35 protons and 33 electrons? Br2+ As2- Br2-