ESAT Chemistry: Atomic structure and the Periodic Table, set 2

• Answer all questions. No calculator.
• Each question has exactly one correct answer.

1. 11 mark

   An atom's nucleus occupies only a tiny fraction of its total volume, yet accounts for almost all of its mass.

   Which of the following correctly explains this?

   1. A Electrons are heavy and confined to the nucleus, while protons and neutrons move in shells around it and contribute little mass.
   2. B Protons, neutrons and electrons all have approximately equal mass, but only the particles in the nucleus are packed closely together.
   3. C Protons and neutrons are relatively heavy and confined to the nucleus, while electrons contribute negligible mass despite occupying the much larger volume of the shells around it.
   4. D The nucleus is large because it contains most of the atom's electrons, which are what give the atom most of its mass.
2. 21 mark

   Which statement about protons and neutrons is correct?

   1. A They have approximately the same relative mass (about 1), but different relative charges: the proton is +1 and the neutron is 0.
   2. B They have the same relative charge (both +1), but different relative masses.
   3. C They carry equal and opposite charges, and the neutron is roughly 2000 times lighter than the proton.
   4. D Both carry zero charge, since only the electron carries any charge within the atom.
3. 31 mark

   A sulfide ion, S2-, is formed from a sulfur atom of atomic number 16 and mass number 32.

   What is the total number of protons, neutrons and electrons in one S2- ion?

   1. A 48
   2. B 46
   3. C 66
   4. D 50
4. 41 mark

   An ion Y2+ is formed from an atom of mass number 24.

   The ion contains 10 electrons. How many neutrons does it contain?

   1. A 10
   2. B 12
   3. C 14
   4. D 16
5. 51 mark

   What is the electron configuration of a Ca2+ ion, formed from a calcium atom of atomic number 20?

   1. A 2,8,8
   2. B 2,8,8,2
   3. C 2,8,8,0
   4. D 2,8,6
6. 61 mark

   Which of the following is the correct electron configuration for potassium (atomic number 19)?

   1. A 2,8,9
   2. B 2,9,8
   3. C 2,8,8,1
   4. D 3,8,8
7. 71 mark

   Copper has atomic number 29 and exists as two isotopes, copper-63 and copper-65.

   How many more neutrons does the copper-65 isotope have than the copper-63 isotope?

   1. A 36
   2. B 0
   3. C 70
   4. D 2
8. 81 mark

   A mass spectrometer analyses a sample of an element of atomic number 10 and records three isotopes: 90% with mass number 20, a negligible trace with mass number 21, and 10% with mass number 22.

   In a sample of 500 atoms with these abundances, how many neutrons in total are contributed by the mass-22 isotope atoms present?

   1. A 1100
   2. B 600
   3. C 6000
   4. D 500
9. 91 mark

   A sample of element R contains only two isotopes, mass numbers 6 and 7, in the ratio 1:3 (isotope-6 to isotope-7).

   Calculate the relative atomic mass of this sample.

   1. A 6.25
   2. B 6.5
   3. C 7
   4. D 6.75
10. 101 mark

    Element G has relative atomic mass 24.5 and exists as only two isotopes, mass numbers 24 and 26.

    What percentage of the sample is the mass-26 isotope?

    1. A 25%
    2. B 50%
    3. C 75%
    4. D 12.5%
11. 111 mark

    Which of the following pairs of elements are both members of the same Period, rather than the same Group?

    1. A Lithium and Sodium
    2. B Fluorine and Chlorine
    3. C Lithium and Fluorine
    4. D Beryllium and Magnesium
12. 121 mark

    Argon has atomic number 18 and a relative atomic mass of approximately 40. Potassium has atomic number 19 and a relative atomic mass of approximately 39.

    In the modern Periodic Table, which element is placed first (earlier), and why?

    1. A Argon is placed first, because elements are arranged by increasing atomic number, and 18 is less than 19, even though argon's relative atomic mass is slightly the greater of the two.
    2. B Potassium is placed first, because elements are arranged by increasing relative atomic mass, and potassium's mass (about 39) is less than argon's (about 40).
    3. C Argon is placed first, because elements are arranged by increasing relative atomic mass, and its mass happens to be the lower of the two once rounded.
    4. D Potassium is placed first, because a Group 1 metal is always placed before a noble gas from the row above it.
13. 131 mark

    Which of the following statements correctly pairs a Group number with the family of elements found in it, using IUPAC numbering (Groups 1-18)?

    1. A Group 18: transition metals
    2. B Group 2: alkaline earth metals
    3. C Group 1: alkaline earth metals
    4. D Group 16: halogens
14. 141 mark

    Which electron configuration belongs to the element found in Period 3, Group 16 of the Periodic Table (IUPAC numbering, Groups 1-18)?

    1. A 2,8,16
    2. B 2,6,8
    3. C 2,8,8
    4. D 2,8,6
15. 151 mark

    Which of the following correctly explains why reactivity increases down Group 1 (the alkali metals)?

    1. A Atomic radius decreases down the group, so the outer electron is held more loosely and is easier to lose.
    2. B The number of protons decreases down the group, which reduces the nuclear charge and makes the outer electron easier to lose.
    3. C Atomic radius increases down the group, so the single outer electron is further from the nucleus and more shielded by inner shells, making it easier to lose.
    4. D Atomic radius increases down the group, so the nucleus exerts a stronger pull on the outer electron, making it easier to lose.

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: C

   1. A proton and a neutron each have a relative mass of about 1, while an electron's relative mass is only about 1/2000 of that, so it is often treated as negligible.
   2. The nucleus holds the protons and neutrons, so it holds almost all of an atom's mass, even though it is tiny compared with the atom as a whole.
   3. The electrons occupy the much larger space of the shells (energy levels) around the nucleus, which is why the atom's volume is mostly empty space patrolled by low-mass electrons.
   4. So the correct explanation is that protons and neutrons are heavy and confined to the nucleus, while electrons contribute negligible mass despite occupying the larger volume around it, which is option C.
   • Why not A: Swaps the particles around: places the heavy electrons inside the nucleus and sends the protons and neutrons, which actually carry almost all the mass, out into the shells.
   • Why not B: Assumes all three sub-atomic particles have approximately the same mass, when an electron's mass is roughly 1/2000 of a proton's or neutron's.
   • Why not D: Confuses electrons with the mass-carrying particles, and wrongly assumes the nucleus itself is the larger region of the atom, when it is the electron shells that occupy most of the volume.

2. Question 2Answer: A

   1. A proton has relative mass 1 and relative charge +1.
   2. A neutron has relative mass approximately 1 (essentially the same as a proton), but relative charge 0, since it carries no charge.
   3. So protons and neutrons share almost the same mass but differ in charge: +1 for the proton and 0 for the neutron.
   4. This matches option A.
   • Why not B: Gives the neutron the proton's own positive charge (+1), when the neutron is actually uncharged (relative charge 0).
   • Why not C: Describes the electron's relationship to the proton (opposite charge, about 1/2000 of the mass), not the neutron's, which has almost the same mass as a proton and no charge at all.
   • Why not D: Wrongly removes the proton's charge as well, when the proton carries relative charge +1; it is only the neutron that is uncharged.

3. Question 3Answer: D

   1. The atomic number gives the proton count directly: 16 protons.
   2. Neutrons = mass number - atomic number = 32 - 16 = 16.
   3. A 2- charge means the ion has gained 2 electrons compared with the neutral atom, so electrons = 16 + 2 = 18.
   4. Total sub-atomic particles = protons + neutrons + electrons = 16 + 16 + 18 = 50, which is option D.
   • Why not A: Treats the ion as if it were the neutral sulfur atom, using 16 electrons and ignoring that the 2- charge means 2 extra electrons have been gained.
   • Why not B: Assumes a negative charge means electrons have been lost, subtracting 2 electrons instead of adding them, when a 2- ion has gained 2 electrons compared with the neutral atom.
   • Why not C: Uses the mass number itself (32) as the neutron count, instead of subtracting the atomic number (32 - 16 = 16) to find the true number of neutrons.

4. Question 4Answer: B

   1. A 2+ charge means the ion has 2 fewer electrons than the neutral atom, so its proton count = electrons + charge = 10 + 2 = 12.
   2. Neutrons = mass number - proton count = 24 - 12 = 12.
   3. So this ion contains 12 neutrons, which is option B.
   • Why not A: Uses the ion's electron count (10) directly as the neutron count, without using the mass number or working out the proton count at all.
   • Why not C: Uses the electron count (10) as if it were the proton count, forgetting that a 2+ ion has 2 fewer electrons than protons: the true proton count is 10 + 2 = 12.
   • Why not D: Treats a 2+ charge as meaning the ion has gained 2 electrons rather than lost them, giving protons = 10 - 2 = 8 instead of 10 + 2 = 12.

5. Question 5Answer: A

   1. A neutral calcium atom (atomic number 20) has the electron configuration 2,8,8,2.
   2. Forming a 2+ ion removes the two outermost electrons, which are the two in the fourth shell.
   3. This leaves the configuration 2,8,8 (18 electrons), with the emptied fourth shell simply not written.
   4. So the Ca2+ ion has configuration 2,8,8, which is option A.
   • Why not B: Gives the neutral calcium atom's configuration, forgetting that forming the 2+ ion removes its outer two electrons.
   • Why not C: Correctly removes the outer two electrons but wrongly keeps the now-empty fourth shell written as a trailing zero, rather than dropping it since it holds no electrons.
   • Why not D: Removes four electrons instead of two, as if the ion's charge were 4+ rather than 2+, leaving only 16 electrons instead of 18.

6. Question 6Answer: C

   1. The first shell holds a maximum of 2 electrons and the second a maximum of 8; for potassium and calcium among the first 20 elements, the third shell also holds only 8 before a new (fourth) shell begins, rather than continuing to its full theoretical capacity.
   2. Potassium has atomic number 19, so 19 electrons must be placed: 2 in the first shell, 8 in the second, 8 in the third (2 + 8 + 8 = 18), leaving 1 electron for a new fourth shell.
   3. So potassium's electron configuration is 2,8,8,1, which is option C.
   • Why not A: Keeps adding electrons to the third shell past its usual limit of 8 for these elements, instead of starting a new fourth shell once the third shell reaches 8.
   • Why not B: Places the 19th electron in the second shell instead of starting a new, fourth shell once the first two shells (2 and 8) and the third shell (8) are full.
   • Why not D: Overfills the innermost shell to 3 electrons, when the first shell holds a maximum of 2.

7. Question 7Answer: D

   1. Neutrons = mass number - atomic number, and both isotopes share copper's atomic number of 29.
   2. Copper-63 has 63 - 29 = 34 neutrons, and copper-65 has 65 - 29 = 36 neutrons.
   3. The difference between the two neutron counts is 36 - 34 = 2.
   4. So copper-65 has 2 more neutrons than copper-63, which is option D.
   • Why not A: Reports the neutron count of the heavier isotope alone (65 - 29 = 36), rather than the difference between the two isotopes' neutron counts.
   • Why not B: Assumes that because both are isotopes of copper, sharing the same atomic number, their neutron counts must be equal, when isotopes are defined by differing neutron numbers despite the shared proton number.
   • Why not C: Adds the two isotopes' neutron counts (34 + 36 = 70) instead of subtracting them to find the difference between them.

8. Question 8Answer: B

   1. Only 10% of the sample is the mass-22 isotope: 10% of 500 = 50 atoms.
   2. Each mass-22 atom has neutrons = mass number - atomic number = 22 - 10 = 12.
   3. Total neutrons contributed by these atoms = 50 x 12 = 600.
   4. So the mass-22 isotope atoms contribute 600 neutrons in total, which is option B.
   • Why not A: Uses the mass number itself (22) as the neutron count, instead of subtracting the atomic number (22 - 10 = 12) first.
   • Why not C: Treats the entire sample of 500 atoms as though every one were the mass-22 isotope, instead of first finding that only 10% (50 atoms) are that isotope.
   • Why not D: Correctly finds 50 mass-22 atoms, but then uses the neutron count of the major (mass-20) isotope, 20 - 10 = 10, instead of the mass-22 isotope's own neutron count of 12.

9. Question 9Answer: D

   1. A ratio of 1:3 means 1 part in 4 is isotope-6 and 3 parts in 4 are isotope-7, so the sample is 25% mass-6 and 75% mass-7.
   2. Relative atomic mass = (0.25 x 6) + (0.75 x 7) = 1.5 + 5.25 = 6.75.
   3. So the relative atomic mass of this sample is 6.75, which is option D.
   • Why not A: Reverses the ratio, treating isotope-6 as the majority isotope (75%) and isotope-7 as the minority (25%), the opposite way round from the 1:3 ratio given.
   • Why not B: Takes a simple average of the two mass numbers, (6 + 7) / 2 = 6.5, ignoring that the isotopes are not present in equal amounts.
   • Why not C: Reports only the mass number of the majority (75%) isotope, ignoring the 25% of the sample that is the lighter, mass-6 isotope.

10. Question 10Answer: A

    1. Let x be the fraction of the sample that is the mass-26 isotope, so (1 - x) is the fraction that is mass-24.
    2. Relative atomic mass = 24(1 - x) + 26x = 24 + 2x.
    3. Setting this equal to 24.5: 24 + 2x = 24.5, so 2x = 0.5, so x = 0.25.
    4. So 25% of the sample is the mass-26 isotope, which is option A.
    • Why not B: Assumes the two isotopes must be equally abundant (50:50) simply because 24.5 lies between 24 and 26, without weighting by how close the relative atomic mass is to each mass number.
    • Why not C: Correctly computes (26 - 24.5) / (26 - 24) = 0.75, but this is the fraction of the lighter (mass-24) isotope, not the mass-26 isotope the question asks for.
    • Why not D: Correctly finds that the mass-26 isotope contributes an excess of 24.5 - 24 = 0.5 out of a total mass-number spread of 2, but then halves this fraction again before converting to a percentage, giving 12.5% instead of 25%.

11. Question 11Answer: C

    1. Periods are the horizontal rows of the Periodic Table, and elements share a Period when their atomic numbers fall within the same row's range.
    2. Lithium (atomic number 3) and fluorine (atomic number 9) both fall within Period 2, which spans atomic numbers 3 to 10.
    3. The other three pairs listed are each two members of the same vertical Group (alkali metals, halogens, or alkaline earth metals), not the same Period.
    4. So the pair that are both in the same Period is lithium and fluorine, which is option C.
    • Why not A: Lithium and sodium are both in Group 1 (the alkali metals), a vertical column of the table, not the same horizontal row (Period).
    • Why not B: Fluorine and chlorine are both in Group 17 (the halogens), a vertical column, not the same Period.
    • Why not D: Beryllium and magnesium are both in Group 2 (the alkaline earth metals), a vertical column, not the same Period.

12. Question 12Answer: A

    1. The modern Periodic Table orders elements by increasing atomic number, not by relative atomic mass.
    2. Argon has atomic number 18 and potassium has atomic number 19, so argon (the lower atomic number) is placed first.
    3. This is despite argon having a slightly greater relative atomic mass (about 40) than potassium (about 39); ordering by mass alone would have placed potassium first, which is exactly the kind of exception that led to atomic number replacing atomic mass as the ordering principle.
    4. So argon is placed first, because the table orders by increasing atomic number, which is option A.
    • Why not B: Orders the elements by relative atomic mass rather than atomic number; this is the historical, pre-1913 approach, and this exact pair is the one whose mass order does not match its atomic-number order.
    • Why not C: Reaches the right element (argon first) but for the wrong reason: it claims argon's mass is the lower of the two, when argon's mass (about 40) is actually the greater one; it is potassium's mass that is lower.
    • Why not D: Invents a rule about Group 1 metals always following noble gases, which is not how the table is actually ordered; the real principle is increasing atomic number.

13. Question 13Answer: B

    1. Group 1 holds the alkali metals and Group 2 holds the alkaline earth metals.
    2. Group 16 holds common non-metals (the oxygen family), Group 17 holds the halogens, and Group 18 holds the noble gases; the transition metals sit in the central block rather than a single numbered main Group.
    3. So the correctly paired statement is Group 2: alkaline earth metals, which is option B.
    • Why not A: Confuses Group 18, home of the noble gases, with the transition metals, which occupy the central block of the table rather than a numbered main Group.
    • Why not C: Names Group 1 as the alkaline earth metals, but Group 1 is actually the alkali metals; the alkaline earth metals are the family found in Group 2.
    • Why not D: Names Group 16 as the halogens, but Group 16 holds common non-metals such as oxygen and sulfur; the halogens are the family found in Group 17.

14. Question 14Answer: D

    1. Period 3 means the atom has 3 occupied electron shells.
    2. For a main-group element in Groups 13 to 18, the number of outer-shell electrons is the Group number minus 10, so Group 16 gives 16 - 10 = 6 outer electrons.
    3. The first two shells fill first (2, then 8), leaving the 6 outer electrons for the third shell: 2,8,6.
    4. So the correct configuration is 2,8,6, which is option D.
    • Why not A: Uses the Group number itself (16) directly as the number of outer-shell electrons, instead of subtracting 10 first (16 - 10 = 6) for a main-group element with Groups numbered 13 to 18.
    • Why not B: Has the right total number of electrons, but places 6 of them in the second shell instead of the third; the second shell must be filled to its maximum of 8 before any electrons go into the third shell.
    • Why not C: Gives the element a full outer shell of 8 electrons, as though it belonged to Group 18 (the noble gases), rather than using the Group 16 given in the question.

15. Question 15Answer: C

    1. Down Group 1, each successive element has one more electron shell, so atomic radius increases.
    2. The single outer electron is therefore further from the nucleus and shielded by more inner shells, both of which weaken the force holding it to the atom.
    3. A more weakly held outer electron is lost more easily, so reactivity increases down the group.
    4. So the correct explanation is that atomic radius increases down the group, weakening the pull on the outer electron, which is option C.
    • Why not A: States that atomic radius decreases down the group, when it actually increases; the outer electron is further away, not closer, in the lower members of the group.
    • Why not B: Claims the number of protons decreases down the group, when atomic number, and so the number of protons, actually increases at every step down a Group.
    • Why not D: Correctly states that atomic radius increases down the group, but then wrongly claims this makes the nuclear pull on the outer electron stronger, when greater distance and increased shielding by inner shells actually weaken that pull.