52 worked answers · short steps first, explanations when you need them
Chapter Prompts, Activities, and Review Questions
Textbook pages 141–161
Question Think It Over 1
Are atoms indivisible? Why do electrons not fall into the nucleus? Why have scientists changed atomic models?
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Atoms contain smaller particles: protons, neutrons, and electrons. In the Bohr model, electrons stay in allowed energy levels and do not lose energy while in them. Scientists change models when new experiments reveal evidence an older model cannot explain.
Answer
Atoms contain smaller particles: protons, neutrons, and electrons. In the Bohr model, electrons stay in allowed energy levels and do not lose energy while in them. Scientists change models when new experiments reveal evidence an older model cannot explain.
Question Pause and Ponder 1
In a clay-and-bead model of Thomson’s atom, what if the positive clay charge is smaller than the total negative charge of the beads? What if the clay itself is negative?
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(i) The model would have a net negative charge, so it would not represent a neutral atom. (ii) Negative clay plus negative beads is still negative; the model would not be neutral.
Answer
(i) The model would have a net negative charge, so it would not represent a neutral atom.
(ii) Negative clay plus negative beads is still negative; the model would not be neutral.
Question Pause and Ponder 2
How is an orange or lemon like Thomson’s model of the atom, and where does the comparison fail?
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The soft pulp can represent the spread-out positive charge and the seeds can represent electrons embedded in it. But fruit is much larger, its seeds are not tiny electrons, and its charges are not real. It is only a simple visual comparison.
Answer
The soft pulp can represent the spread-out positive charge and the seeds can represent electrons embedded in it. But fruit is much larger, its seeds are not tiny electrons, and its charges are not real. It is only a simple visual comparison.
Question Pause and Ponder 3
Why did Thomson conclude that electrons are present in all atoms?
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Cathode rays behaved the same when Thomson changed the cathode material and the gas in the tube. He concluded that the negative particles in the rays—electrons—are parts of every kind of atom.
Answer
Cathode rays behaved the same when Thomson changed the cathode material and the gas in the tube. He concluded that the negative particles in the rays—electrons—are parts of every kind of atom.
Question Think as a Scientist
What would likely happen if Rutherford’s gold foil were thicker?
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More alpha particles would pass through more layers of atoms, so the chance of being deflected would increase. Most would still pass through, but more could be deflected or scattered backward after coming close to a nucleus.
Answer
More alpha particles would pass through more layers of atoms, so the chance of being deflected would increase. Most would still pass through, but more could be deflected or scattered backward after coming close to a nucleus.
Question Pause and Ponder 4
What might happen if negatively charged particles were used instead of alpha particles in Rutherford’s experiment?
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The negative particles would be attracted to the positively charged nucleus and could bend toward it. Their paths would depend on their speed and distance from the nucleus; the result would not be the same repulsion seen for positive alpha particles.
Answer
The negative particles would be attracted to the positively charged nucleus and could bend toward it. Their paths would depend on their speed and distance from the nucleus; the result would not be the same repulsion seen for positive alpha particles.
Question Pause and Ponder 5
Why did the few alpha particles that bounced back challenge Thomson’s model?
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Thomson’s positive charge was spread throughout the atom, so it could not provide a tiny, concentrated obstacle strong enough to send a heavy alpha particle backward. The result supported a small, dense, positively charged nucleus.
Answer
Thomson’s positive charge was spread throughout the atom, so it could not provide a tiny, concentrated obstacle strong enough to send a heavy alpha particle backward. The result supported a small, dense, positively charged nucleus.
Question Pause and Ponder 6
Suggest one question to ask Rutherford about his gold foil experiment.
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Example: “How did you estimate the size of the nucleus from the paths of the scattered alpha particles?” Answers may vary.
Answer
Example: “How did you estimate the size of the nucleus from the paths of the scattered alpha particles?” Answers may vary.
Question Pause and Ponder 7
Assertion: Rutherford concluded that most atomic mass is concentrated in a tiny central region. Reason: Thomson pictured electrons embedded in a uniformly positive sphere. Choose the correct relation.
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Option (ii): both statements are true, but the reason does not explain the assertion. Rutherford inferred the concentrated nucleus from the alpha-particle scattering results.
Answer
Option (ii): both statements are true, but the reason does not explain the assertion. Rutherford inferred the concentrated nucleus from the alpha-particle scattering results.
Question Activity 8.1
What is the approximate number of atoms across a 0.1 mm thick sheet of paper if one atom is about 10⁻¹⁰ m wide?
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Convert 0.1 mm to 10⁻⁴ m. Number of atoms ≈ 10⁻⁴ ÷ 10⁻¹⁰ = 10⁶. About one million atoms fit across the thickness.
Answer
Convert 0.1 mm to 10⁻⁴ m. Number of atoms ≈ 10⁻⁴ ÷ 10⁻¹⁰ = 10⁶. About one million atoms fit across the thickness.
Question Pause and Ponder 8
Invent a new element name and choose a symbol that follows the symbol-writing rules.
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Example: “Gauravium”, symbol Gv. The first letter is capital and the second is lowercase. In reality, only IUPAC approves official element names and symbols.
Answer
Example: “Gauravium”, symbol Gv. The first letter is capital and the second is lowercase. In reality, only IUPAC approves official element names and symbols.
Question Pause and Ponder 9
What problems would arise if scientists used different symbols for the same element?
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Scientists could misunderstand formulas, experiments, and results. One shared symbol system makes scientific communication clear across languages and countries.
Answer
Scientists could misunderstand formulas, experiments, and results. One shared symbol system makes scientific communication clear across languages and countries.
Question Pause and Ponder 10
How are atomic number, protons, and electrons related in a neutral atom?
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Atomic number equals the number of protons. A neutral atom has equal numbers of protons and electrons, so its electron count also equals its atomic number.
Answer
Atomic number equals the number of protons. A neutral atom has equal numbers of protons and electrons, so its electron count also equals its atomic number.
Question Activity 8.2
How can you build a simple model to show the first eighteen elements?
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Use a nucleus with the correct number of protons, then place electrons in shells: up to 2 in K, then up to 8 in L, and then up to 8 in M for the first eighteen elements. Label the element and atomic number. The model is a simplified picture.
Answer
Use a nucleus with the correct number of protons, then place electrons in shells: up to 2 in K, then up to 8 in L, and then up to 8 in M for the first eighteen elements. Label the element and atomic number. The model is a simplified picture.
Question Pause and Ponder 11
What are the combining capacities of nitrogen in NH₃ and magnesium in MgCl₂?
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Nitrogen combines with three hydrogen atoms, so its combining capacity is 3. Magnesium combines with two chlorine atoms, so its combining capacity is 2.
Answer
Nitrogen combines with three hydrogen atoms, so its combining capacity is 3. Magnesium combines with two chlorine atoms, so its combining capacity is 2.
Question Activity 8.3
Use shell arrangements to find common valencies of the first eighteen elements.
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For atomic numbers 1–18, the common valencies are: H 1, He 0, Li 1, Be 2, B 3, C 4, N 3, O 2, F 1, Ne 0, Na 1, Mg 2, Al 3, Si 4, P 3, S 2, Cl 1, Ar 0. Noble gases have full outer shells, so their common valency is 0.
Answer
For atomic numbers 1–18, the common valencies are: H 1, He 0, Li 1, Be 2, B 3, C 4, N 3, O 2, F 1, Ne 0, Na 1, Mg 2, Al 3, Si 4, P 3, S 2, Cl 1, Ar 0. Noble gases have full outer shells, so their common valency is 0.
Question Pause and Ponder 12
Do isotopes of one element have the same number of electrons?
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Neutral isotopes have the same number of electrons because they have the same number of protons. Their neutron numbers differ.
Answer
Neutral isotopes have the same number of electrons because they have the same number of protons. Their neutron numbers differ.
Question Pause and Ponder 13
Why does a neutron change an atom’s mass number but not its atomic number?
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Mass number counts protons plus neutrons, so adding a neutron raises it by 1. Atomic number counts only protons, so it stays unchanged.
Answer
Mass number counts protons plus neutrons, so adding a neutron raises it by 1. Atomic number counts only protons, so it stays unchanged.
Question Pause and Ponder 14
Why do electrons not fall into the nucleus according to Bohr’s model?
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Bohr proposed that electrons can occupy only certain allowed energy levels. While an electron stays in one of these levels, it does not lose energy, so it does not spiral into the nucleus.
Answer
Bohr proposed that electrons can occupy only certain allowed energy levels. While an electron stays in one of these levels, it does not lose energy, so it does not spiral into the nucleus.
Question 1
Which statements about Rutherford’s gold foil experiment are correct? Explain why the other statements are incorrect.
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(ii) and (iii) are correct. The experiment supports a small, positively charged nucleus.
Answer
(ii) and (iii) are correct. The experiment supports a small, positively charged nucleus.
Question 2
Mark each statement about Bohr’s model correct or incorrect and explain.
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(i), (ii), and (iv) are incorrect; (iii) is correct.
Answer
(i) ,
(ii) , and
(iv) are incorrect; (iii) is correct.
Question 3
Nuclei X, Y, and Z have proton counts 18, 17, 17 and neutron counts 19, 18, 20. Compare (i) Y and Z, and (ii) Z and X.
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Y and Z are isotopes; Z and X are isobars.
Answer
Y and Z are isotopes; Z and X are isobars.
Question 4
What did large-angle deflections and backward-scattering of a few alpha particles show about the positive part of an atom?
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Positive charge and most atomic mass are concentrated in a tiny, dense nucleus.
Answer
Positive charge and most atomic mass are concentrated in a tiny, dense nucleus.
Question 5
Put Dalton’s, Thomson’s, Rutherford’s, and Bohr’s models in chronological order and describe each change.
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Dalton → Thomson → Rutherford → Bohr.
Answer
Dalton → Thomson → Rutherford → Bohr.
Question 6
Electrons move around a nucleus. Why do they not fly away? What keeps them attracted?
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Electrostatic attraction keeps negative electrons bound to the positive nucleus.
Answer
Electrostatic attraction keeps negative electrons bound to the positive nucleus.
Question 7
Assertion: discovering subatomic particles helped explain atomic structure. Reason: a neutral atom has equal numbers of electrons and protons. Choose the correct option.
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Option (ii): both statements are true, but R does not explain A.
Answer
Option (ii): both statements are true, but R does not explain A.
Question 8
For magnesium-24 (atomic number 12), find its protons, neutrons, electrons, and shell arrangement.
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Magnesium has 12 protons, 12 neutrons, and 12 electrons; its arrangement is 2, 8, 2.
Answer
Magnesium has 12 protons, 12 neutrons, and 12 electrons; its arrangement is 2, 8, 2.
Question 9
For each of the four shell diagrams in Fig. 8.17, give element, symbol, total electrons, valence electrons, valency, protons, and atomic number.
Why could Rutherford’s model not explain stability, while Bohr’s model could?
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Rutherford’s model could not explain stable electron orbits. Bohr’s fixed energy levels did.
Answer
Rutherford’s model could not explain stable electron orbits. Bohr’s fixed energy levels did.
Question 11
An atom written as ⁷⁰₃₁X has 31 electrons. How many neutrons are in its nucleus?
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39 neutrons.
Answer
39 neutrons.
Question 12
An atom has 79 protons and mass number 197. Find its neutrons and electrons.
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118 neutrons and 79 electrons.
Answer
118 neutrons and 79 electrons.
Question 13
Complete the five rows with atomic number/mass number: (5,–); (–,14) Nitrogen with 7 electrons; (–,24) with 12 protons; (15,–); (–,1) with zero neutrons.
An atom has atomic number 26 and 56 nucleons. Find its electrons, protons, and neutrons.
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A neutral atom has 26 electrons and 26 protons because atomic number equals proton count. Neutrons = 56 − 26 = 30.
Answer
A neutral atom has 26 electrons and 26 protons because atomic number equals proton count. Neutrons = 56 − 26 = 30.
Question Pause and Ponder 11
A nucleus contains 20 protons and has mass number 41. How many neutrons does it contain?
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Neutrons = mass number − protons = 41 − 20 = 21.
Answer
Neutrons = mass number − protons = 41 − 20 = 21.
Question Pause and Ponder 12
An atom has 18 neutrons and atomic number 17. What is its mass number?
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Atomic number 17 means 17 protons. Mass number = protons + neutrons = 17 + 18 = 35.
Answer
Atomic number 17 means 17 protons. Mass number = protons + neutrons = 17 + 18 = 35.
Question Pause and Ponder 13
An atom written as ²³A has 11 electrons. Find its neutrons.
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Assuming a neutral atom, it has 11 protons. Neutrons = mass number − protons = 23 − 11 = 12.
Answer
Assuming a neutral atom, it has 11 protons. Neutrons = mass number − protons = 23 − 11 = 12.
Question Activity 8.4
How many electrons can fit in the K, L, and M shells, and how are they filled?
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The 2n² rule gives maximum capacities K (n=1): 2; L (n=2): 8; M (n=3): 18. Electrons fill from the shell closest to the nucleus outward; for the first 18 elements, the outer shell holds at most 8.
Answer
The 2n² rule gives maximum capacities K (n=1): 2; L (n=2): 8; M (n=3): 18. Electrons fill from the shell closest to the nucleus outward; for the first 18 elements, the outer shell holds at most 8.
Question Pause and Ponder 14
How are the two electrons of helium arranged in its shells?
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Both electrons go into the first (K) shell, which can hold up to two electrons.
Answer
Both electrons go into the first (K) shell, which can hold up to two electrons.
Question Pause and Ponder 15
What is the maximum number of electrons allowed in an atom’s outermost shell in the simple model used here?
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The outermost shell can hold up to 8 electrons. The first shell is an exception and holds up to 2.
Answer
The outermost shell can hold up to 8 electrons. The first shell is an exception and holds up to 2.
Question Pause and Ponder 16
How can atoms of one element have different numbers of neutrons, and what changes as a result?
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Atoms of the same element always have the same proton count, but they may have different neutron counts. Their mass numbers differ; these atoms are isotopes.
Answer
Atoms of the same element always have the same proton count, but they may have different neutron counts. Their mass numbers differ; these atoms are isotopes.
Question Pause and Ponder 17
What do we call atoms with the same mass number but different atomic numbers?
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They are isobars: atoms of different elements with the same mass number and different proton counts.
Answer
They are isobars: atoms of different elements with the same mass number and different proton counts.
Question Pause and Ponder 18
Why do isotopes of the same element have similar chemical properties?
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They have the same number of protons and, when neutral, the same number and arrangement of electrons. Chemical behaviour mainly depends on electrons, especially valence electrons.
Answer
They have the same number of protons and, when neutral, the same number and arrangement of electrons. Chemical behaviour mainly depends on electrons, especially valence electrons.
Question Pause and Ponder 19
Two isotopes of bromine are ⁷⁹Br (49.7%) and ⁸¹Br (50.3%). Calculate the average atomic mass.
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Average mass = (79 × 49.7/100) + (81 × 50.3/100) = 39.263 + 40.743 = 80.006 u, or about 80.0 u.
Answer
Average mass = (79 × 49.7/100) + (81 × 50.3/100) = 39.263 + 40.743 = 80.006 u, or about 80.0 u.
Question Pause and Ponder 20
The isotope average for chlorine can be calculated as a weighted average. What does a weighted average mean, and why is it used?
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Multiply each isotope’s mass by its fraction in nature, then add the results. This gives more weight to the isotope that is more common. For chlorine: 35 × 0.75 + 37 × 0.25 = 35.5 u.
Answer
Multiply each isotope’s mass by its fraction in nature, then add the results. This gives more weight to the isotope that is more common. For chlorine: 35 × 0.75 + 37 × 0.25 = 35.5 u.
Question 14
Element X has mass number 35 and 18 neutrons. Find its protons/electrons, atomic number, name, electron arrangement, valence electrons, mass number after adding two neutrons, and relation to the new atom.
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X is chlorine: 17 protons/electrons, shells 2,8,7. With two extra neutrons, it is chlorine-37.
Answer
X is chlorine: 17 protons/electrons, shells 2,8,7. With two extra neutrons, it is chlorine-37.
Question 15
An atom has 12 protons and 12 neutrons. If each electron were replaced by a particle with the same charge but 500 times the mass, what changes in (i) atomic number, (ii) atomic mass, (iii) mass number, and (iv) overall charge?
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Atomic number stays 12; atomic mass increases; mass number stays 24; the atom remains neutral.
Answer
Atomic number stays 12; atomic mass increases; mass number stays 24; the atom remains neutral.
Question Project 1
How could you make an Atomic Prediction Board game using atomic clues?
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Make cards with clues such as proton number, mass number, neutron count, electron count, and valency. Players use the clues to name the element and check their answer against an answer key. Award a point for each correct prediction.
Answer
Make cards with clues such as proton number, mass number, neutron count, electron count, and valency. Players use the clues to name the element and check their answer against an answer key. Award a point for each correct prediction.
Question Project 2
Prepare a short report on how atomic properties affect healthcare, energy, agriculture, and technology.
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Example points: radioisotopes help diagnose or treat disease; uranium isotopes can fuel nuclear power; isotopes help date ancient remains; the arrangement of electrons explains how materials conduct electricity and form chemical bonds.
Answer
Example points: radioisotopes help diagnose or treat disease; uranium isotopes can fuel nuclear power; isotopes help date ancient remains; the arrangement of electrons explains how materials conduct electricity and form chemical bonds.
Question Project 3
How can a short play tell the story of atomic models and the scientists who developed them?
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Give students roles such as Dalton, Thomson, Rutherford, Bohr, and Chadwick. Each scientist presents the evidence for their model and explains what new discovery changed the earlier idea. End by showing that scientific models improve when new evidence appears.
Answer
Give students roles such as Dalton, Thomson, Rutherford, Bohr, and Chadwick. Each scientist presents the evidence for their model and explains what new discovery changed the earlier idea. End by showing that scientific models improve when new evidence appears.
Question Project 4
What should an animation of the different atomic models show?
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Show Dalton’s solid atom, Thomson’s positive sphere with embedded electrons, Rutherford’s small nucleus and mostly empty space, and Bohr’s fixed energy levels. Label the models as simplified scientific ideas, not to-scale pictures.
Answer
Show Dalton’s solid atom, Thomson’s positive sphere with embedded electrons, Rutherford’s small nucleus and mostly empty space, and Bohr’s fixed energy levels. Label the models as simplified scientific ideas, not to-scale pictures.
Question Project 5
After watching a documentary about atoms, what should a short report include?
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Write the documentary title and main idea; describe what it taught about atomic structure; name scientists and their contributions; mention the most interesting part and one question you still have. Answers depend on the documentary watched.
Answer
Write the documentary title and main idea; describe what it taught about atomic structure; name scientists and their contributions; mention the most interesting part and one question you still have. Answers depend on the documentary watched.
Question Project 6
How can you draw a bar graph showing electron counts in energy levels for three elements?
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Choose three elements, write their shell arrangements, then draw one bar per shell for each element. For example, Na is 2, 8, 1; Mg is 2, 8, 2; and Cl is 2, 8, 7. Label the horizontal axis with shells and the vertical axis with number of electrons.
Answer
Choose three elements, write their shell arrangements, then draw one bar per shell for each element. For example, Na is 2, 8, 1; Mg is 2, 8, 2; and Cl is 2, 8, 7. Label the horizontal axis with shells and the vertical axis with number of electrons.