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HKDSE Chemistry Recap: Atoms, Isotopes and the Periodic Table | Vanilla (2026)

HKDSE Chemistry — Atoms, Isotopes and the Periodic Table

Subject: Chemistry

Vanilla (2026)

Issue date: Wednesday, 14 October 2026

Total: 70 marks  |  Suggested time: 100–110 minutes  |  Topic: atoms and isotopes, relative isotopic and atomic masses, electronic arrangement, periods and groups

Instructions

  1. Question 1 consists of 10 multiple-choice questions. Each has one correct answer. Write your answers in the answer boxes at the end of Question 1.
  2. In Question 2, state whether each statement is True or False. If a statement is false, rewrite it correctly.
  3. Questions 3 to 7 are structured questions. Show your working clearly and give numerical answers with appropriate units where needed.
  4. The letters used for elements in the questions are not the actual symbols, unless stated otherwise.
  5. Unless other values are given, take the relative isotopic mass of an isotope to be equal to its mass number.
  6. A Periodic Table may be used. The atomic numbers of some elements are given below.

Paper structure

Question Type Marks Score
1 Multiple choice (10 questions) 10
2 True or false (10 statements) 10
3 Comprehensive structured question 1 10
4 Comprehensive structured question 2 10
5 Comprehensive structured question 3 10
6 Comprehensive structured question 4 10
7 Comprehensive structured question 5 10
Total 70
Data — atomic numbers: H = 1, He = 2, Li = 3, Be = 4, B = 5, C = 6, N = 7, O = 8, F = 9, Ne = 10, Na = 11, Mg = 12, Al = 13, Si = 14, P = 15, S = 16, Cl = 17, Ar = 18, K = 19, Ca = 20.

Total score: / 70

01

Section A — Atoms, Isotopes and the Periodic Table

Multiple Choice

Each question has one correct answer. Choose the best option. (1 mark each)

  1. Which of the following species has the same number of electrons as an atom of 40Ar and also has more neutrons than an atom of 40Ar? (Atomic number of Ar = 18)
    1. 39K+
    2. 37Cl−
    3. 42Ca2+
    4. 44Ca2+
  2. Element L has three isotopes, 24L, 25L and 26L. The percentage abundance of 24L is 80% and the relative atomic mass of L is 24.25. Taking the relative isotopic masses to be equal to the mass numbers, what is the percentage abundance of 26L?
    1. 5%
    2. 10%
    3. 15%
    4. 20%
  3. Among the first 20 elements in the Periodic Table, how many have atoms in which the number of electrons in the outermost shell is equal to the number of occupied electron shells?
    1. 2
    2. 3
    3. 4
    4. 5
  4. Which of the following pairs of elements (given by their atomic numbers) are in the same group of the Periodic Table?
    • (1) 4 and 20
    • (2) 2 and 12
    • (3) 9 and 17
    1. (1) and (2) only
    2. (2) and (3) only
    3. (1), (2) and (3)
    4. (1) and (3) only
  5. Small pieces of three different Group I metals P, Q and R, of similar size, are dropped separately onto water. The observations are shown below.
    Metal Observations
    P Melts into a silvery ball, moves rapidly on the water and burns with a lilac flame
    Q Does not melt, moves slowly on the water and gives bubbles steadily
    R Melts into a silvery ball, moves rapidly on the water and does not burn
    Which of the following shows P, Q and R in order of increasing atomic number?
    1. P < Q < R
    2. Q < R < P
    3. R < P < Q
    4. Q < P < R
  6. Hydrogen has three isotopes, 1H, 2H and 3H, and chlorine has two isotopes, 35Cl and 37Cl. Using mass numbers, how many different values of relative molecular mass are possible for HCl molecules?
    1. 3
    2. 4
    3. 5
    4. 6
  7. Which of the following comparisons of metallic character (tendency to lose electrons) are correct?
    • (1) The element with atomic number 12 is more metallic than the element with atomic number 20.
    • (2) The element with atomic number 13 is less metallic than the element with atomic number 12.
    • (3) The element with atomic number 19 is more metallic than the element with atomic number 12.
    1. (1) and (2) only
    2. (1) and (3) only
    3. (2) and (3) only
    4. (1), (2) and (3)
  8. Fig. 1 shows part of the Periodic Table. The letters are not the actual symbols of the elements. Which of the following statements are correct?
    Group I II III IV V VI VII 0 Period 2 Period 3 Period 4 A B C D E F G H I
    Fig. 1 — Part of the Periodic Table (letters are not the actual symbols)
    • (1) A and I have similar chemical properties.
    • (2) The atomic radius of E is smaller than that of D.
    • (3) An atom of I has one more occupied electron shell than an atom of H.
    1. (1) and (2) only
    2. (1) and (3) only
    3. (2) and (3) only
    4. (1), (2) and (3)
  9. Element Z has two isotopes, 63Z and 65Z. The relative atomic mass of Z is 63.55. Which of the following statements are correct?
    • (1) No atom of Z has a relative mass of exactly 63.55.
    • (2) There are more atoms of 63Z than atoms of 65Z in a natural sample of Z.
    • (3) Atoms of 65Z have more electrons than atoms of 63Z.
    1. (1) only
    2. (2) and (3) only
    3. (1) and (3) only
    4. (1) and (2) only
  10. Four elements P, Q, R and S have consecutive atomic numbers, and the atomic number increases in the order P, Q, R, S. R is a Period 3 element whose atoms have very little tendency to gain, lose or share electrons. Which of the following statements are correct? (The letters are not the actual symbols.)
    • (1) Atoms of P have six electrons in the outermost shell.
    • (2) Q and S are in the same period.
    • (3) S is a metal and its atoms have one electron in the outermost shell.
    1. (1) and (2) only
    2. (1) and (3) only
    3. (2) and (3) only
    4. (1), (2) and (3)
1
2
3
4
5
6
7
8
9
10
10 marks
02

Section B — Atoms, Isotopes and the Periodic Table

True or False

For each statement, state whether it is True or False. If it is false, rewrite the statement correctly. (1 mark each)

  1. An atom of 40Ca and a 40Ca2+ ion are isotopes of calcium because they have the same number of protons.
  2. The relative isotopic mass of a 12C atom is exactly 12 by definition, whereas the relative atomic mass of carbon is 12.01 because natural carbon also contains a small proportion of heavier isotopes such as 13C.
  3. An atom of the hydrogen isotope 3H contains 1 proton, 2 neutrons and 1 electron, and it has the same chemical properties as an atom of 1H.
  4. A calcium atom has 20 electrons. Since the third shell can hold up to 18 electrons, the electronic arrangement of a calcium atom is 2,8,10.
  5. The period number of an element equals the number of electrons in its outermost shell, whereas the group number of an element in Groups I to VII equals the number of occupied electron shells in its atoms.
  6. Helium is placed in Group 0 because, like the other noble gases, its atoms have eight electrons in the outermost shell.
  7. Across a period from left to right, the atomic radius generally decreases because the nuclear charge increases while the electrons are added to the same outermost shell, so the outermost electrons are attracted more strongly by the nucleus.
  8. Going down Group VII, the reactivity of the halogens decreases because their atoms lose electrons less readily.
  9. An element has two isotopes with relative isotopic masses 10.01 and 11.01, and its relative atomic mass is 10.81. Therefore the isotope with relative isotopic mass 11.01 is more abundant than the other isotope.
  10. A sodium atom loses one electron to form a Na+ ion, which has the same electronic arrangement as argon, the noble gas in the same period.
10 marks
03

Section C — Comprehensive Structured Question 1

Isotopes of Magnesium and Relative Atomic Mass

Magnesium (atomic number 12) occurs naturally as a mixture of three isotopes, 24Mg, 25Mg and 26Mg. Fig. 2 shows the percentage abundance of each isotope. The value for 26Mg is not shown. The relative isotopic masses of the isotopes are given in the table.

100 80 60 40 20 0 79.0 10.0 ? 24 25 26 Percentage abundance (%) Mass number of the isotope
Fig. 2 — Percentage abundance of the isotopes of magnesium
Isotope 24Mg 25Mg 26Mg
Relative isotopic mass 23.99 24.99 25.98

(a)

  1. Explain why 24Mg, 25Mg and 26Mg are isotopes of the same element. (1 mark)
  2. State the number of protons, neutrons and electrons in one 26Mg2+ ion. (1 mark)

(b) Deduce the percentage abundance of 26Mg, and hence calculate the relative atomic mass of magnesium. Give your answer to 4 significant figures. (3 marks)

(c) A student calculates the relative atomic mass of magnesium as (24 + 25 + 26) ÷ 3 = 25.0. Explain why this method is wrong and why the relative atomic mass of magnesium is much closer to 24 than to 25. (1 mark)

(d) A sample contains 2000 magnesium atoms with the natural isotopic composition shown in Fig. 2. Calculate the total number of neutrons in the sample. (2 marks)

(e) Element X is in Group VII and Period 4. It has only two isotopes, 79X and 81X, and its relative atomic mass is 79.90. Taking the relative isotopic masses to be equal to the mass numbers, calculate the percentage abundance of 79X. (2 marks)

10 marks
04

Section C — Comprehensive Structured Question 2

Electronic Arrangement and the Position of Elements

The table below gives information about the atoms of six elements A, B, C, D, E and F. All six elements are among the first 20 elements of the Periodic Table. The letters are not the actual symbols of the elements.

Element Information
A The atom has three occupied electron shells. The number of electrons in its outermost shell is half the number of electrons in its second shell.
B The ion B2+ has the same electronic arrangement as an atom of neon.
C The number of occupied electron shells in the atom is equal to the number of electrons in its outermost shell. The atomic number of C is greater than 10.
D The ion D− has the same electronic arrangement as an atom of argon.
E The atom has four occupied electron shells and has the same number of electrons in its outermost shell as an atom of B.
F The atom has fewer electrons than an atom of A, but twice as many electrons in its outermost shell as an atom of A.

(a) Complete the table below to show the electronic arrangement of the atom, the group and the period of each element. (3 marks)

Element Electronic arrangement of the atom Group Period
A
B
C
D
E
F

(b) Which two of the elements A to F are in the same group? Explain why they are expected to have similar chemical properties. (2 marks)

(c) A, B and C are in the same period. Arrange A, B and C in order of increasing atomic radius, and explain your answer. (3 marks)

(d) Atoms of D and E each form a monatomic ion. Write the formulae of these two ions (using the letters D and E), and state the noble gas whose atoms have the same electronic arrangement as both ions. (1 mark)

(e) Explain why element F is chemically unreactive. (1 mark)

10 marks
05

Section C — Comprehensive Structured Question 3

Periods and Groups: Variation of Atomic Radius

Fig. 3 shows how the atomic radius varies with atomic number for some of the first 20 elements. The noble gases (atomic numbers 10 and 18) are omitted. Straight lines join the points for elements in the same period, and dashed lines join the last point of one period to the first point of the next.

0.25 0.20 0.15 0.10 0.05 0 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Atomic radius / nm Atomic number
Fig. 3 — Atomic radius against atomic number (noble gases omitted)

(a)

  1. Describe the trend in atomic radius across Period 3 (atomic numbers 11 to 17). (1 mark)
  2. Explain this trend in terms of atomic structure. (2 marks)

(b) The atomic radius rises sharply between atomic numbers 9 and 11, and again between atomic numbers 17 and 19. Explain these sharp rises in terms of electronic arrangement. (2 marks)

(c) The atom with atomic number 19 has a greater nuclear charge than the atom with atomic number 11, but its atomic radius is larger. Explain. (2 marks)

(d) An atom with atomic number 17 has three occupied electron shells, whereas an atom with atomic number 3 has only two. However, the atom with atomic number 17 has the smaller atomic radius. Explain this apparent contradiction. (2 marks)

(e) Of the elements shown in Fig. 3, which one has atoms that lose electrons most readily? Give a reason. (1 mark)

10 marks
06

Section C — Comprehensive Structured Question 4

Groups: Predicting the Properties of Rubidium and Astatine

Part 1 — Group I. The table shows some data about the Group I elements. The elements are named in this question.

Element Atomic number Atomic radius / nm Melting point / °C
Lithium 3 0.152 181
Sodium 11 0.186 98
Potassium 19 0.227 63
Rubidium 37 ? ?
Caesium 55 0.265 29

(a) Using the trends in the table, predict a value for the atomic radius of rubidium and a value for its melting point. (2 marks)

(b)

  1. Rubidium reacts with water in a similar way to potassium. Write the chemical equation for the reaction, with state symbols. (1 mark)
  2. Rubidium reacts with water more vigorously than potassium does. Explain, in terms of atomic structure. (2 marks)

Part 2 — Group VII. Astatine (atomic number 85) is the Group VII element below iodine. Some properties of the other halogens are shown below.

Halogen Atomic number State at room temperature and pressure Colour
Chlorine 17 Gas Greenish-yellow
Bromine 35 Liquid Reddish-brown
Iodine 53 Solid Purple-black

(c)

  1. Predict the state and the colour of astatine at room temperature and pressure. (1 mark)
  2. Aqueous chlorine is added to aqueous sodium astatide, NaAt. State whether a displacement reaction occurs, and write the ionic equation for any reaction. (2 marks)
  3. By considering atomic structure, explain why the reactivity of the halogens decreases down the group. (2 marks)
10 marks
07

Section C — Comprehensive Structured Question 5

Group 0: Isotopes and Electronic Arrangement of the Noble Gases

Neon (Ne, atomic number 10) is a Group 0 element in Period 2. Natural neon consists of three isotopes, 20Ne, 21Ne and 22Ne. The percentage abundance of 21Ne is 0.2% and the relative atomic mass of neon is 20.18. Take the relative isotopic masses to be equal to the mass numbers.

(a) For an atom of 22Ne, state the number of protons, neutrons and electrons, and write its electronic arrangement. (2 marks)

(b) Calculate the percentage abundances of 22Ne and 20Ne in natural neon. (3 marks)

(c) Neon exists as single atoms and is chemically unreactive. Explain, in terms of electronic arrangement. (2 marks)

(d) Atoms of helium and atoms of magnesium both have two electrons in the outermost shell, but helium is placed in Group 0 and magnesium in Group II. Explain. (2 marks)

(e) Which two of the following ions have the same electronic arrangement as an atom of argon? (1 mark)

K+     Mg2+     Cl−     O2−

10 marks

Question 1 — Multiple Choice

1. D
2. A
3. B
4. D
5. B
6. C
7. C
8. A
9. D
10. B
  1. D — 44Ca2+. An atom of 40Ar has 18 electrons and 40 − 18 = 22 neutrons. All four ions have 18 electrons (K+: 19 − 1; Cl−: 17 + 1; Ca2+: 20 − 2). Their numbers of neutrons are: 39K, 20; 37Cl, 20; 42Ca, 22; 44Ca, 24. Only 44Ca2+ has more than 22 neutrons. (42Ca2+ has the same number, not more.)
  2. A — 5%. Let the abundance of 25L be x%. Then the abundance of 26L is (20 − x)%. 24(80) + 25x + 26(20 − x) = 24.25 × 100, so 2440 − x = 2425 and x = 15. The abundance of 26L is 20 − 15 = 5%. (15% is the abundance of 25L.)
  3. B — 3. The elements are H (1; one shell, 1 outermost electron), Be (2,2) and Al (2,8,3). No other element among the first 20 satisfies the condition: for example He has one shell but 2 electrons, Li (2,1) has 2 shells but 1 electron, and K (2,8,8,1) and Ca (2,8,8,2) have 4 shells.
  4. D — (1) and (3) only. (1) Be (2,2) and Ca (2,8,8,2) both have 2 outermost electrons and are in Group II. (2) He is in Group 0 and Mg (2,8,2) is in Group II, so they are not in the same group, although both atoms have 2 outermost electrons. (3) F (2,7) and Cl (2,8,7) are both in Group VII.
  5. B — Q < R < P. Reactivity of Group I metals with water increases down the group. Q (does not melt, slow bubbling) is lithium, R (melts, moves rapidly, no flame) is sodium and P (burns with a lilac flame) is potassium. The order of increasing atomic number is Li < Na < K, i.e. Q < R < P.
  6. C — 5. There are 3 × 2 = 6 types of molecule, with relative molecular masses 1 + 35 = 36, 1 + 37 = 38, 2 + 35 = 37, 2 + 37 = 39, 3 + 35 = 38 and 3 + 37 = 40. The value 38 occurs twice (1H37Cl and 3H35Cl), so there are only 5 different values: 36, 37, 38, 39 and 40.
  7. C — (2) and (3) only. Metallic character increases down a group and decreases across a period from left to right. (1) is wrong: element 20 (Ca) is below element 12 (Mg) in Group II, so Ca is more metallic. (2) is correct: element 13 (Al) is to the right of element 12 (Mg) in Period 3. (3) is correct: element 19 (K) is both to the left of and below Mg.
  8. A — (1) and (2) only. (1) A (Group II) and I (Group II) are in the same group, so they have similar chemical properties. (2) D and E are both in Period 3 and E is to the right of D, so E has the larger nuclear charge and the smaller atomic radius. (3) is wrong: H and I are both in Period 4, so their atoms have the same number of occupied shells (four).
  9. D — (1) and (2) only. (1) is correct: the relative atomic mass is a weighted average, and the relative isotopic masses of the two isotopes are close to 63 and 65, so no atom has a relative mass of 63.55. (2) is correct: 63.55 is closer to 63 than to 65 (it is below the midpoint, 64), so 63Z is the more abundant isotope. (3) is wrong: isotopes of an element have the same number of electrons (and protons).
  10. B — (1) and (3) only. R is a Period 3 element with very little tendency to gain, lose or share electrons, so R is argon (atomic number 18). Then P has atomic number 16 (2,8,6), Q has 17 (2,8,7) and S has 19 (2,8,8,1). (1) is correct. (2) is wrong: Q (Cl) is in Period 3 but S (K) is in Period 4. (3) is correct: S is potassium, a Group I metal with 1 outermost electron.

Question 2 — True or False

  1. False. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. 40Ca and 40Ca2+ have the same number of neutrons (20) and the same mass number. They are the same nuclide (an atom and its ion), not isotopes.
  2. True. The relative atomic mass scale is based on the 12C atom, so its relative isotopic mass is exactly 12. The relative atomic mass of carbon is the weighted average, which is slightly above 12 because of 13C.
  3. True. The atomic number of H is 1, so 3H has 1 proton and 1 electron, and 3 − 1 = 2 neutrons. It has the same electronic arrangement as 1H and so the same chemical properties.
  4. False. The electronic arrangement of a calcium atom is 2,8,8,2. For the first 20 elements, the third shell takes 8 electrons before the fourth shell starts to be filled (K is 2,8,8,1 and Ca is 2,8,8,2).
  5. False. The statement is the wrong way round. The period number equals the number of occupied electron shells, and the group number (Groups I to VII) equals the number of electrons in the outermost shell.
  6. False. Helium atoms have two electrons in the outermost shell (the first shell is full, a stable duplet). Helium is in Group 0 because it is chemically unreactive. The other noble gases have eight outermost electrons.
  7. True. The number of occupied shells stays the same across a period while the nuclear charge increases, so the outermost electrons are attracted more strongly and the atom becomes smaller.
  8. False. Halogen atoms react by gaining an electron. The reactivity decreases down the group because the atoms gain an electron less readily (the incoming electron enters a shell farther from the nucleus and more shielded, so it is attracted less strongly).
  9. True. Let the abundance of the heavier isotope be x: 10.01(1 − x) + 11.01x = 10.81, so 10.01 + x = 10.81 and x = 0.80. The heavier isotope is 80% abundant. (The relative atomic mass 10.81 is much closer to 11.01 than to 10.01.)
  10. False. Na+ (2,8) has the same electronic arrangement as neon, the noble gas in the preceding period (Period 2). Argon is 2,8,8.

Question 3 — Isotopes of Magnesium and Relative Atomic Mass

(a) Isotopes

(i) They are atoms of the same element, with the same number of protons (12) but different numbers of neutrons (12, 13 and 14), i.e. different mass numbers. (1)
(ii) 26Mg2+: 12 protons, 14 neutrons, 10 electrons (1)

(b) Relative atomic mass of magnesium

Abundance of 26Mg = 100 − 79.0 − 10.0 = 11.0% (1)
Relative atomic mass = 23.99 × 79.0 + 24.99 × 10.0 + 25.98 × 11.0100 (1)
= 1895.21 + 249.9 + 285.78100 = 24.31 (1)

(c) The student’s method

A simple average of the mass numbers assumes that the three isotopes are equally abundant. The relative atomic mass is a weighted average that takes the relative abundances into account, and 24Mg is by far the most abundant isotope (79.0%), so the value is close to 24. (1)

(d) Neutrons in 2000 atoms

Numbers of atoms: 24Mg, 0.790 × 2000 = 1580; 25Mg, 0.100 × 2000 = 200; 26Mg, 0.110 × 2000 = 220 (1)
Number of neutrons = 1580 × 12 + 200 × 13 + 220 × 14
= 18 960 + 2600 + 3080 = 24 640 (1)

(e) Element X

Let the percentage abundance of 79X be x. Then 81X is (100 − x)%.
79x + 81(100 − x)100 = 79.90 (1)
8100 − 2x = 7990, so x = 55% (81X is 45%) (1)
Check: 0.55 × 79 + 0.45 × 81 = 43.45 + 36.45 = 79.90. Average number of neutrons per Mg atom in (d) = 24 640 ÷ 2000 = 12.32, which is consistent with a relative atomic mass of 24.31 (24.31 − 12 = 12.31).
Common errors: (1) forgetting to deduce the abundance of 26Mg before calculating; (2) using the mass numbers instead of the relative isotopic masses in (b); (3) giving the answer in (b) to the wrong number of significant figures; (4) in (d), multiplying the number of atoms by the mass number instead of the number of neutrons.

Question 4 — Electronic Arrangement and the Position of Elements

(a) Deducing the elements

Element Electronic arrangement Group Period Reasoning
A 2,8,4 IV 3 Second shell is full (8), so outermost shell has 8 ÷ 2 = 4 electrons.
B 2,8,2 II 3 B2+ is 2,8, so the atom has 10 + 2 = 12 electrons.
C 2,8,3 III 3 Number of shells = outermost electrons, atomic number > 10: three shells, 3 electrons.
D 2,8,7 VII 3 D− is 2,8,8, so the atom has 18 − 1 = 17 electrons.
E 2,8,8,2 II 4 Four shells and 2 outermost electrons (same as B).
F 2,8 0 2 Eight outermost electrons and fewer than 14 electrons in total.
1 mark for all six electronic arrangements correct; 1 mark for all six groups correct; 1 mark for all six periods correct. (3)

(b) Elements in the same group

B and E (1)
Their atoms have the same number of electrons (2) in the outermost shell, so they react in a similar way (both lose 2 electrons to form 2+ ions). (1)

(c) Atomic radius of A, B and C

Order of increasing atomic radius: A < C < B (1)
A, B and C are in the same period, so their atoms have the same number of occupied shells (three). (1)
The nuclear charge increases from B (+12) to C (+13) to A (+14), so the outermost electrons are attracted more strongly and the atom is smaller. (1)

(d) Ions of D and E

D− and E2+; both have the electronic arrangement 2,8,8 of an atom of argon. (1)

(e) Unreactivity of F

F is neon. Its atom has a stable electronic arrangement with 8 electrons in the outermost shell (a full outermost shell), so it has very little tendency to gain, lose or share electrons. (1)
Common errors: (1) writing 2,8,10 for calcium; (2) stating that “F is in Group VIII”; (3) stating that elements with the same number of outermost electrons are always in the same group (compare He with Be and Mg); (4) explaining the trend in (c) only by “more electrons” without referring to nuclear charge.

Question 5 — Periods and Groups: Variation of Atomic Radius

(a) Trend across Period 3

(i) The atomic radius decreases from atomic number 11 to 17 (from about 0.186 nm to about 0.099 nm). (1)
(ii) Across the period, the atoms have the same number of occupied electron shells (three), but the nuclear charge increases. (1)
The outermost electrons are in the same shell and are attracted more strongly by the nucleus, so the atoms become smaller. (1)

(b) Sharp rises

Atomic number 9 (F) is 2,7 and atomic number 11 (Na) is 2,8,1. [Likewise 17 is 2,8,7 and 19 is 2,8,8,1.] The outermost electron of the atom with atomic number 11 (or 19) is in a new, higher electron shell (the third, or fourth, shell). (1)
This shell is farther from the nucleus and the outermost electron is shielded by the additional filled inner shell, so the atom is much larger. (1)

(c) Atomic numbers 19 and 11 (down Group I)

The atom with atomic number 19 (K, 2,8,8,1) has four occupied shells, one more than the atom with atomic number 11 (Na, 2,8,1). (1)
Its outermost electron is farther from the nucleus and more shielded by the inner electrons; this effect outweighs the increase in nuclear charge. (1)

(d) Atomic numbers 17 and 3

The nuclear charge of the atom with atomic number 17 (+17) is much greater than that of the atom with atomic number 3 (+3), so its outermost electrons are attracted much more strongly by the nucleus. (1)
It lies at the right-hand end of Period 3, where the radius has already decreased greatly across the period (0.186 nm to 0.099 nm), and this decrease outweighs the effect of the extra shell. (1)

(e) Easiest loss of electrons

The element with atomic number 19 (potassium): it has the largest atomic radius, so its single outermost electron is farthest from the nucleus and attracted least strongly. (1)
Marking note: For (a)(ii), do not accept “more electrons” as the explanation; the key point is the increase in nuclear charge with the number of shells unchanged. For (b) and (c), accept “extra electron shell” or “extra energy level”.
Common errors: (1) saying that the atomic radius decreases because “electrons are lost”; (2) forgetting to mention that the number of occupied shells is the same in (a); (3) in (c), saying that the nuclear charge decreases down a group; (4) in (d), ignoring the nuclear charge.

Question 6 — Groups: Predicting the Properties of Rubidium and Astatine

(a) Predictions for rubidium

Atomic radius: any value between 0.227 nm and 0.265 nm (e.g. 0.24–0.25 nm) (1)
Melting point: any value between 29 °C and 63 °C (e.g. about 40–50 °C) (1)

(b) Reaction of rubidium with water

(i) 2Rb(s) + 2H2O(l) → 2RbOH(aq) + H2(g) (1)
(ii) A rubidium atom has more occupied electron shells than a potassium atom, so its outermost electron is farther from the nucleus and more shielded by the inner electrons. (1)
The outermost electron is attracted less strongly by the nucleus, so it is lost more easily and rubidium is more reactive. (1)

(c) Astatine and the halogens

(i) Solid, black (dark / purple-black). Both the state and the colour are needed. (1)
(ii) A displacement reaction occurs, because chlorine is more reactive than astatine. (1)
Cl2(aq) + 2At−(aq) → 2Cl−(aq) + At2(aq) (1)
(iii) A halogen atom reacts by gaining one electron into its outermost shell. Going down the group, the atoms have more occupied electron shells and are larger. (1)
The incoming electron enters a shell farther from the nucleus and more shielded, so it is attracted less strongly; the atoms gain an electron less readily and the reactivity decreases. (1)
Marking note: In (a), do not give the mark if the predicted value is outside the stated range. In (c)(ii), the equation must be balanced and use ionic species (Cl− and At−); do not accept the unbalanced equation or one that includes Na+ spectator ions.
Common errors: (1) applying the Group I explanation (“loses electrons more easily”) to the halogens in (c)(iii); (2) predicting that astatine is a gas or a liquid; (3) writing the equation in (b)(i) without state symbols or with H2O(aq); (4) giving a melting point for rubidium that is higher than that of potassium.

Question 7 — Group 0: Isotopes and Electronic Arrangement of the Noble Gases

(a) An atom of 22Ne

10 protons, 12 neutrons, 10 electrons (1)
Electronic arrangement: 2,8 (1)

(b) Percentage abundances

Let the percentage abundance of 22Ne be y. Then 20Ne is (99.8 − y)%.
20(99.8 − y) + 21(0.2) + 22y100 = 20.18 (1)
2000.2 + 2y = 2018, so y = 8.9% (22Ne) (1)
20Ne = 99.8 − 8.9 = 90.9% (1)
Check: 0.909 × 20 + 0.002 × 21 + 0.089 × 22 = 18.18 + 0.042 + 1.958 = 20.18.

(c) Neon is monatomic and unreactive

A neon atom has a stable electronic arrangement (2,8) with a full outermost shell of 8 electrons. (1)
It therefore has very little tendency to gain, lose or share electrons, so it does not combine with other atoms and exists as single atoms. (1)

(d) Helium and magnesium

A helium atom has only one occupied shell, and the first shell is full with 2 electrons (a stable duplet). It has very little tendency to gain or lose electrons, so it behaves like the other noble gases and is placed in Group 0. (1)
A magnesium atom (2,8,2) has an outermost shell that is not full. It tends to lose the 2 outermost electrons to attain a stable arrangement, so it is reactive and belongs to Group II. (1)

(e) Ions with the electronic arrangement of argon

K+ and Cl− (both 2,8,8) (1)
(Mg2+ and O2− are both 2,8, the same as neon.)
Common errors: (1) forgetting the 0.2% of 21Ne when setting up the equation in (b); (2) stating that all noble gases have 8 outermost electrons; (3) in (d), saying that helium is unreactive simply because “it has 2 outermost electrons”, which also applies to magnesium; (4) writing the electronic arrangement of Mg2+ as 2,8,2.
Mark weighting: Section A (10) + Section B (10) + Question 3 (10) + Question 4 (10) + Question 5 (10) + Question 6 (10) + Question 7 (10) = 70 marks.
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