ESAT Chemistry: Atomic structure and the Periodic Table, set 1

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

1. 11 mark

   Which statement correctly describes the structure of an atom?

   1. A A central nucleus containing protons and neutrons, surrounded by electrons moving in shells.
   2. B A central nucleus containing protons and electrons, surrounded by neutrons moving in shells.
   3. C A central nucleus containing only neutrons, surrounded by protons and electrons moving in shells.
   4. D A central nucleus containing protons only, with neutrons and electrons both moving in shells.
2. 21 mark

   Which row correctly states the relative charge and relative mass of an electron, using proton mass = 1 as the reference?

   1. A Relative charge -1, relative mass very small (about 1/2000 of a proton).
   2. B Relative charge +1, relative mass very small (about 1/2000 of a proton).
   3. C Relative charge 0, relative mass very small (about 1/2000 of a proton).
   4. D Relative charge -1, relative mass 1.
3. 31 mark

   An atom of chlorine can be written as chlorine-35, with atomic number 17 and mass number 35.

   How many neutrons does one atom of chlorine-35 contain?

   1. A 35
   2. B 18
   3. C 52
   4. D 17
4. 41 mark

   An aluminium ion, Al3+, is formed from an aluminium atom of atomic number 13 and mass number 27.

   How many electrons does this Al3+ ion contain?

   1. A 10
   2. B 13
   3. C 16
   4. D 14
5. 51 mark

   Magnesium has atomic number 12.

   What is the electron configuration of a magnesium atom, written in the standard comma-separated form?

   1. A 2,10
   2. B 2,8,1,1
   3. C 8,2,2
   4. D 2,8,2
6. 61 mark

   An atom has the electron configuration 2,8,7.

   What is its atomic number?

   1. A 15
   2. B 17
   3. C 16
   4. D 18
7. 71 mark

   Four atoms are described by their numbers of protons and neutrons: Atom 1 has 6 protons and 6 neutrons; Atom 2 has 6 protons and 8 neutrons; Atom 3 has 7 protons and 7 neutrons; Atom 4 has 8 protons and 8 neutrons.

   Which two atoms are isotopes of each other?

   1. A Atom 1 and Atom 2
   2. B Atom 3 and Atom 4
   3. C Atom 2 and Atom 4
   4. D Atom 2 and Atom 3
8. 81 mark

   A mass spectrometer analyses a sample of boron (atomic number 5) and finds two isotopes: 20% with mass number 10, and 80% with mass number 11.

   How many neutrons are in the more abundant isotope of boron?

   1. A 11
   2. B 16
   3. C 6
   4. D 5
9. 91 mark

   A sample of chlorine contains 75% chlorine-35 atoms and 25% chlorine-37 atoms.

   Calculate the relative atomic mass of this sample of chlorine.

   1. A 36.5
   2. B 35.5
   3. C 35.0
   4. D 36.0
10. 101 mark

    Element X exists as two isotopes: 80% with mass number 20, and 20% with mass number 25.

    Calculate the relative atomic mass of element X.

    1. A 20
    2. B 21
    3. C 22.5
    4. D 25
11. 111 mark

    In the Periodic Table, what are the vertical columns and the horizontal rows called?

    1. A The columns are called Periods and the rows are called Groups.
    2. B Both the columns and the rows are called Periods.
    3. C The columns are called Groups and the rows are called Periods.
    4. D Both the columns and the rows are called Groups.
12. 121 mark

    The modern Periodic Table arranges elements in order of increasing:

    1. A relative atomic mass
    2. B number of electron shells
    3. C melting point
    4. D atomic number
13. 131 mark

    Using IUPAC numbering, in which Group of the Periodic Table are the halogens found?

    1. A Group 18
    2. B Group 17
    3. C Group 1
    4. D Group 2
14. 141 mark

    An atom has the electron configuration 2,8,3.

    In which Group and Period of the Periodic Table, using IUPAC numbering (Groups 1-18), is this element found?

    1. A Group 3, Period 13
    2. B Group 13, Period 3
    3. C Group 3, Period 3
    4. D Group 8, Period 3
15. 151 mark

    Which statement correctly describes the trend in reactivity down Group 1 and down Group 17 of the Periodic Table?

    1. A Reactivity increases down Group 1 and decreases down Group 17.
    2. B Reactivity decreases down both Group 1 and Group 17.
    3. C Reactivity increases down both Group 1 and Group 17.
    4. D Reactivity decreases down Group 1 and increases down Group 17.

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: A

   1. An atom's mass is concentrated in a central nucleus, which contains the protons and the neutrons.
   2. The electrons carry very little mass compared with the nucleus and move around it in shells (energy levels), not inside it.
   3. So the nucleus holds protons and neutrons, with electrons in shells surrounding it, which is option A.
   • Why not B: Swaps electrons and neutrons, placing electrons in the nucleus and neutrons in the shells instead of the other way round.
   • Why not C: Leaves protons out of the nucleus entirely, as if only neutrons were found there.
   • Why not D: Leaves neutrons out of the nucleus entirely, placing them in shells alongside the electrons.

2. Question 2Answer: A

   1. The three sub-atomic particles are compared using relative charge and relative mass, taking a proton's mass as 1.
   2. A proton has relative charge +1 and relative mass 1; a neutron has relative charge 0 and relative mass 1.
   3. An electron has relative charge -1, the same size as the proton's charge but opposite in sign, while its relative mass is very small, about 1/2000 of a proton's mass, so it is often treated as negligible.
   4. So the electron's correct row is charge -1 with a very small (negligible) relative mass, which is option A.
   • Why not B: Gives the electron the correct (negligible) mass but assigns it the proton's positive charge instead of a negative charge.
   • Why not C: Gives the electron the correct (negligible) mass but treats it as uncharged, like a neutron, instead of carrying charge -1.
   • Why not D: Gives the electron the correct charge but assumes it has the same relative mass as a proton (mass 1), when its mass is negligible.

3. Question 3Answer: B

   1. The mass number (35) is the total count of protons plus neutrons in the nucleus.
   2. The atomic number (17) is the number of protons.
   3. Number of neutrons = mass number - atomic number = 35 - 17 = 18.
   4. So this atom of chlorine contains 18 neutrons, which is option B.
   • Why not A: Uses the mass number itself as the neutron count, without subtracting the atomic number.
   • Why not C: Adds the mass number and the atomic number (35 + 17) instead of subtracting one from the other.
   • Why not D: Uses the atomic number (the proton count) as if it were the neutron count.

4. Question 4Answer: A

   1. A neutral aluminium atom has 13 protons and, since it is uncharged, also 13 electrons.
   2. The 3+ charge on this ion means it has lost 3 electrons compared with the neutral atom.
   3. Number of electrons = number of protons - charge = 13 - 3 = 10.
   4. So the Al3+ ion contains 10 electrons, which is option A.
   • Why not B: Assumes the ion has as many electrons as protons, ignoring that losing three electrons is what creates the 3+ charge.
   • Why not C: Adds the charge to the proton count (13 + 3) instead of subtracting it, as if a positive ion had gained electrons.
   • Why not D: Calculates mass number minus atomic number (27 - 13), which gives the number of neutrons, not the number of electrons.

5. Question 5Answer: D

   1. Electron shells fill from the one nearest the nucleus outward, and the first three shells hold a maximum of 2, 8 and 8 electrons respectively.
   2. Magnesium has atomic number 12, so it has 12 electrons to place.
   3. The first shell takes 2, the second shell takes its maximum of 8, leaving 12 - 2 - 8 = 2 electrons for the third shell.
   4. So the configuration is 2,8,2, which is option D.
   • Why not A: Fills the second shell beyond its maximum capacity of 8 electrons, instead of starting a new shell once the second is full.
   • Why not B: Splits the final two electrons across two separate extra shells instead of placing both together in the third shell.
   • Why not C: Lists the shells in reverse order, starting from the outermost shell instead of the one nearest the nucleus.

6. Question 6Answer: B

   1. The atomic number equals the total number of electrons in a neutral atom, which is the sum of the numbers in its electron configuration.
   2. Adding the shells: 2 + 8 + 7 = 17.
   3. So this atom has atomic number 17, which is option B.
   • Why not A: Omits the innermost shell of 2 electrons entirely and adds only the outer two values (8 + 7).
   • Why not C: Misreads the outer shell as holding 6 electrons rather than the stated 7 (2 + 8 + 6).
   • Why not D: Assumes the outer shell must be full at 8 electrons rather than using the stated value of 7 (2 + 8 + 8).

7. Question 7Answer: A

   1. Isotopes are atoms of the same element, so they must have the same number of protons; only their neutron number (and so mass number) differs.
   2. Atom 1 has 6 protons and Atom 2 also has 6 protons, so these two are the same element with different neutron numbers (6 and 8): they are isotopes.
   3. Atom 2 and Atom 3 share a mass number (14) but have different proton numbers (6 and 7), so they are different elements, not isotopes, despite the matching mass number.
   4. So the pair that are isotopes of each other is Atom 1 and Atom 2, which is option A.
   • Why not B: These have different proton numbers and different neutron numbers, so there is no basis for treating them as isotopes of each other.
   • Why not C: Both have 8 neutrons, but different proton numbers (6 and 8); matching neutron numbers alone does not make atoms isotopes.
   • Why not D: Both have mass number 14, but different proton numbers (6 and 7); this is a coincidence of mass number, not the shared proton number that defines isotopes.

8. Question 8Answer: C

   1. The more abundant isotope is boron-11, with mass number 11, since it has the 80% abundance.
   2. Boron's atomic number is 5, so this isotope has 5 protons.
   3. Number of neutrons = mass number - atomic number = 11 - 5 = 6.
   4. So the more abundant isotope of boron contains 6 neutrons, which is option C.
   • Why not A: Uses the mass number itself as the neutron count, without subtracting the atomic number.
   • Why not B: Adds the mass number and atomic number (11 + 5) instead of subtracting one from the other.
   • Why not D: Mistakes the neutron count for the atomic (proton) number.

9. Question 9Answer: B

   1. Relative atomic mass is a weighted average of the isotope masses, weighted by their percentage abundance.
   2. Chlorine-35 contributes 75% of the sample: 0.75 x 35 = 26.25.
   3. Chlorine-37 contributes 25% of the sample: 0.25 x 37 = 9.25.
   4. Adding the contributions: 26.25 + 9.25 = 35.5.
   5. So the relative atomic mass of this sample of chlorine is 35.5, which is option B.
   • Why not A: Swaps the two percentages, weighting chlorine-37 as though it were the major isotope instead of chlorine-35.
   • Why not C: Uses only the mass number of the more abundant isotope (35), ignoring the 25% of the sample that is chlorine-37.
   • Why not D: Takes a simple, unweighted average of 35 and 37, ignoring that the two isotopes are not present in equal amounts.

10. Question 10Answer: B

    1. Relative atomic mass is the percentage-weighted average of the isotope mass numbers, not a simple average of the two masses.
    2. The mass-20 isotope contributes 80%: 0.8 x 20 = 16.
    3. The mass-25 isotope contributes 20%: 0.2 x 25 = 5.
    4. Adding the contributions: 16 + 5 = 21.
    5. So the relative atomic mass of element X is 21, which is option B.
    • Why not A: Uses only the mass number of the more abundant isotope, ignoring the 20% present at mass number 25.
    • Why not C: Takes a simple, unweighted average of 20 and 25, ignoring that the two abundances are not equal.
    • Why not D: Picks the higher mass number regardless of it belonging to the minority isotope.

11. Question 11Answer: C

    1. In the Periodic Table, the vertical columns collect elements with similar chemical properties and are called Groups.
    2. The horizontal rows, across which properties change progressively, are called Periods.
    3. So the columns are Groups and the rows are Periods, which is option C.
    • Why not A: Swaps the two terms, calling the columns Periods and the rows Groups, the reverse of the correct convention.
    • Why not B: Uses the term Periods for both, dropping the term Groups altogether.
    • Why not D: Uses the term Groups for both, dropping the term Periods altogether.

12. Question 12Answer: D

    1. The modern Periodic Table, since Moseley's work in 1913, arranges elements by increasing atomic number, not atomic mass.
    2. Ordering by atomic number places every element so that each has one more proton than the element before it, which fixes the small number of pairs where atomic mass order would have given a different sequence.
    3. So elements are ordered by increasing atomic number, which is option D.
    • Why not A: This was the historical, pre-1913 ordering principle, which for a small number of element pairs gives a different order from the modern one.
    • Why not B: Confuses the number of electron shells, which sets an element's Period, with the quantity used to order the whole table.
    • Why not C: Melting point has no systematic relationship to an element's position in the table.

13. Question 13Answer: B

    1. Using IUPAC numbering, the columns of the Periodic Table are numbered 1 to 18 from left to right.
    2. Group 1 holds the alkali metals and Group 2 the alkaline earth metals; Group 18, on the far right, holds the noble gases.
    3. The halogens sit in the second column from the right, which is Group 17.
    4. So the halogens are in Group 17, which is option B.
    • Why not A: Confuses the halogens with the noble gases, which occupy Group 18, immediately to their right.
    • Why not C: Confuses the halogens with the alkali metals, which occupy Group 1.
    • Why not D: Confuses the halogens with the alkaline earth metals, which occupy Group 2.

14. Question 14Answer: B

    1. The number of occupied electron shells gives the Period number, so the configuration 2,8,3 (three shells) places this element in Period 3.
    2. Under IUPAC numbering (Groups 1-18), a main-group element's Group number is linked to its outer-shell electron count, but for elements with 3 to 8 outer electrons this means adding 10 to that count rather than using it directly.
    3. This atom's outer shell holds 3 electrons, so its Group number is 3 + 10 = 13, not 3.
    4. So this element is in Group 13, Period 3, which is option B.
    • Why not A: Transposes the two values, swapping the Group number and the Period number.
    • Why not C: Gets the Period right but uses the outer-shell electron count directly as the Group number, the pre-IUPAC convention rather than the 1-18 numbering used here.
    • Why not D: Gets the Period right but uses the electron count of the middle shell (8) for the Group number instead of the outer shell (3).

15. Question 15Answer: A

    1. Down Group 1 (the alkali metals), the outer electron is progressively further from the nucleus and more shielded by inner shells, so it is lost more easily and reactivity increases.
    2. Down Group 17 (the halogens), the atom needs to attract an extra electron to react, and the increasing atomic radius and shielding make this harder, so reactivity decreases.
    3. So reactivity increases down Group 1 but decreases down Group 17, which is option A.
    • Why not B: Applies the halogens' decreasing trend to Group 1 as well, when reactivity increases down a metal Group.
    • Why not C: Assumes the metal trend also applies to the halogens, when reactivity actually decreases down a non-metal Group.
    • Why not D: Reverses both trends compared with the correct pattern.