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

HKDSE Chemistry — Atoms, Isotopes, Atomic Mass and the Periodic Table

Subject: Chemistry

Vanilla (2026)

Issue date: Thursday, 8 October 2026

Total: 70 marks  |  Suggested time: 90–100 minutes  |  A Periodic Table is required. Take the relative isotopic mass of carbon-12 as exactly 12. Letters used to represent elements are not chemical symbols. Give numerical answers to 3 significant figures unless stated otherwise.

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. What are the numbers of protons, neutrons and electrons in one ion of 3115P3−?
    1. 15 protons, 16 neutrons, 12 electrons
    2. 15 protons, 31 neutrons, 18 electrons
    3. 15 protons, 16 neutrons, 18 electrons
    4. 18 protons, 16 neutrons, 15 electrons
  2. The table shows the mass numbers and the numbers of neutrons of four neutral atoms W, X, Y and Z.
    Atom Mass number Number of neutrons
    W2412
    X2613
    Y2513
    Z2714
    Which pair of atoms are isotopes of the same element?
    1. W and X
    2. W and Y
    3. X and Y
    4. Y and Z
  3. Which pair of particles contains the same number of electrons?
    1. 2713Al3+ and 199F−
    2. 147N and 168O2−
    3. 3919K+ and 2311Na+
    4. 3216S and 3517Cl−
  4. Chlorine has two isotopes, 3517Cl and 3717Cl. Which of the following statements are correct?
    (1) A 3517Cl atom and a 3717Cl atom have the same number of protons.
    (2) A 3517Cl− ion has the same number of neutrons as a 3717Cl atom.
    (3) A 3517Cl− ion and a 3717Cl− ion have the same electronic arrangement.
    1. (1) and (2) only
    2. (1) and (3) only
    3. (2) and (3) only
    4. (1), (2) and (3)
  5. Which atom contains the same number of neutrons as an atom of 199F?
    1. 168O
    2. 2311Na
    3. 2713Al
    4. 2010Ne
  6. The mass of one carbon-12 atom is 1.993 × 10−23 g. The mass of one atom of an isotope of element Q is 3.321 × 10−23 g. What is the relative isotopic mass of this isotope of Q?
    1. 10.0
    2. 20.0
    3. 40.0
    4. 1.67
  7. Silicon has three naturally occurring isotopes, as shown in the table. What is the relative atomic mass of silicon, correct to 3 significant figures?
    Isotope Relative isotopic mass Percentage abundance
    Silicon-2828.092.2%
    Silicon-2929.04.7%
    Silicon-3030.03.1%
    1. 28.0
    2. 28.3
    3. 29.0
    4. 28.1
  8. An element G has two isotopes, G-69 (relative isotopic mass 69.0) and G-71 (relative isotopic mass 71.0). In a natural sample of G, the ratio of the number of G-69 atoms to the number of G-71 atoms is 3 : 2. What is the relative atomic mass of G?
    1. 69.0
    2. 69.4
    3. 69.8
    4. 70.0
  9. A hypothetical element E has three isotopes, E-50, E-52 and E-54, with relative isotopic masses of 50.0, 52.0 and 54.0 respectively. In a natural sample of E, 40.0% of the atoms are E-52, and the relative atomic mass of E is 52.2. What is the percentage abundance of E-54?
    1. 15%
    2. 25%
    3. 35%
    4. 45%
  10. Which of the following statements about relative atomic mass are correct?
    (1) The relative atomic mass of an element is a whole number if the relative isotopic mass of each of its isotopes is a whole number.
    (2) The relative atomic mass of an element which has only one naturally occurring isotope is equal to the relative isotopic mass of that isotope.
    (3) The relative atomic mass of an element which has two or more naturally occurring isotopes lies between the smallest and the largest relative isotopic masses of its isotopes.
    1. (1) and (2) only
    2. (1) and (3) only
    3. (2) and (3) only
    4. (1), (2) and (3)
  11. Fig. 1 shows the electronic arrangement of a neutral atom of element Q. Each dot (●) represents an electron. What are the atomic number, group and period of Q?
    Fig. 1 — The central circle is the nucleus; each dot is an electron
    1. Atomic number 16, Group VI, Period 3
    2. Atomic number 16, Group III, Period 6
    3. Atomic number 6, Group VI, Period 3
    4. Atomic number 8, Group VI, Period 2
  12. The ion J2+ has the electronic arrangement 2, 8, 8. What is the electronic arrangement of a neutral atom of J?
    1. 2, 8, 6
    2. 2, 8, 8
    3. 2, 8, 10
    4. 2, 8, 8, 2
  13. An ion Y3− contains 18 electrons and 16 neutrons. Which of the following statements is correct?
    1. Y is in Group V and Period 3 of the Periodic Table.
    2. The mass number of Y is 34.
    3. A neutral atom of Y has 18 electrons.
    4. A neutral atom of Y has the electronic arrangement 2, 8, 8.
  14. An atom of element M is in Period 3 of the Periodic Table. The number of electrons in its outermost shell is the same as the number of electrons in its first shell. What is the atomic number of M?
    1. 4
    2. 12
    3. 14
    4. 20
  15. Fig. 2 shows part of the Periodic Table. Only some elements are represented, by letters which are not chemical symbols. The Roman numerals and 0 are group numbers. What is the total number of electrons in the outermost shells of one atom each of X, Y and Z?
    I II III IV V VI VII 0
    Period 1 Z
    Period 2 RXU
    Period 3 SYTL
    Period 4 W
    Fig. 2 — Part of the Periodic Table (columns are groups, rows are periods)
    1. 9
    2. 10
    3. 11
    4. 17
  16. Refer to Fig. 2. Each ion below is formed from an atom of the element shown in Fig. 2 and carries the charge indicated. Which ion has the same electronic arrangement as an atom of L?
    1. T−
    2. U−
    3. X3−
    4. R2+
  17. Refer to Fig. 2. Which of the following statements is correct?
    1. U and X have similar chemical properties because they are in the same period.
    2. S and Y have similar chemical properties because their atoms have the same number of occupied electron shells.
    3. Z and L have similar chemical properties because their atoms have the same number of electrons in the outermost shell.
    4. R and S have similar chemical properties because their atoms have the same number of electrons in the outermost shell.
  18. Which of the following statements about the periods of the Periodic Table are correct?
    (1) Atoms of elements in the same period have the same number of occupied electron shells.
    (2) Across Period 3 from sodium to argon, the number of electrons in the outermost shell of the atoms increases by one from one element to the next.
    (3) Period 2 contains eight elements.
    1. (1) and (2) only
    2. (1) and (3) only
    3. (2) and (3) only
    4. (1), (2) and (3)
  19. Fluorine (atomic number 9) and chlorine (atomic number 17) are in the same group of the Periodic Table. Which of the following statements are correct?
    (1) Atoms of both elements have 7 electrons in the outermost shell.
    (2) A chlorine atom has one more occupied electron shell than a fluorine atom.
    (3) A chlorine atom has more electrons in the outermost shell than a fluorine atom.
    1. (1) and (2) only
    2. (1) and (3) only
    3. (2) and (3) only
    4. (1), (2) and (3)
  20. A particle K+ contains 18 electrons and 20 neutrons. In which group and period of the Periodic Table is element K found?
    1. Group I, Period 3
    2. Group I, Period 4
    3. Group II, Period 4
    4. Group II, Period 3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
20 marks
02

Section B — Comprehensive Structured Question 1

Atoms, Ions and Isotopes

The table shows the numbers of protons, neutrons and electrons in six particles P, Q, R, S, T and U. Each particle is a single atom or a single monatomic ion. (The letters are not chemical symbols.) Use the Periodic Table where necessary.

Particle Number of protons Number of neutrons Number of electrons
P8810
Q91010
R101010
S111210
T101210
U121410

(a) Which two of the particles are isotopes of each other? Explain your answer. (2 marks)

(b) Write the nuclide symbol, in the form AZE with the charge (if any) at the top right, for particle P and for particle U. (2 marks)

(c)

  1. All six particles have the same electronic arrangement. State this electronic arrangement. (1 mark)
  2. Which of the six particles are neutral atoms? (1 mark)

(d) A student says, “P and Q are isotopes of each other because they have the same number of electrons.” Comment on this statement. (2 marks)

(e) An ion W3+ has a mass number of 27 and contains 14 neutrons.

  1. State the number of protons and the number of electrons in one W3+ ion. (1 mark)
  2. State the electronic arrangement of a neutral atom of W. (1 mark)
10 marks
03

Section B — Comprehensive Structured Question 2

Relative Isotopic Mass and Relative Atomic Mass of Neon

Neon (atomic number 10) has three naturally occurring isotopes. Fig. 3 shows their relative abundances in natural neon. Take the relative isotopic masses of the isotopes to be 20.0, 21.0 and 22.0. The mass of one carbon-12 atom is 1.993 × 10−23 g.

0 20 40 60 80 100 90.5% 0.3% 9.2% 20 21 22 Relative isotopic mass Relative abundance (%)
Fig. 3 — Relative abundances of the isotopes of neon

(a) State the meaning of relative isotopic mass. (1 mark)

(b) Calculate the mass, in g, of one atom of neon-22. (1 mark)

(c) Using the data in Fig. 3, calculate the relative atomic mass of neon, correct to 3 significant figures. (3 marks)

(d) State the number of protons, the number of neutrons and the number of electrons in one atom of neon-22. (1 mark)

(e) A sample of neon has been treated so that it contains only neon-20 and neon-22, with 80.0% of the atoms being neon-20.

  1. Calculate the relative atomic mass of neon in this sample. (1 mark)
  2. Explain why this value is different from your answer to (c). (1 mark)

(f) State the electronic arrangement of a neon atom, and the group and period of neon in the Periodic Table. (2 marks)

10 marks
04

Section B — Comprehensive Structured Question 3

Electronic Arrangement: Identifying Five Elements

Elements A, B, C, D and E are among the first 20 elements of the Periodic Table. (The letters are not chemical symbols.) The following information is known.

  • Atoms of A have three occupied electron shells and two electrons in the outermost shell.
  • The ion B− has the same electronic arrangement as an argon atom (atomic number 18).
  • Atoms of C have two occupied electron shells. The number of electrons in the outermost shell is exactly half of the atomic number.
  • D is in Group III and in the same period as B.
  • E is in the same group as A. An atom of E has one more occupied electron shell than an atom of A.

(a) Complete the table. (5 marks, 1 mark for each element)

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

(b) In the box below, draw the electronic arrangement of the ion B−. Use circles for the shells and dots (●) for the electrons, and write the number of protons in the nucleus. (1 mark)

(c) Would you expect atoms of A and atoms of C to have similar chemical properties? Explain your answer. (2 marks)

(d) A helium atom has only 2 electrons in its outermost shell, but helium is placed in Group 0 together with neon, whose atoms have 8 outermost electrons, and not in the same group as C. Explain why. (2 marks)

10 marks
05

Section B — Comprehensive Structured Question 4

Periods and Groups of the Periodic Table

Fig. 4 shows part of the Periodic Table. Some elements are represented by the letters Q, R, S, T, U, W, X, Y and Z, which are not chemical symbols. The Roman numerals and 0 are group numbers.

I II III IV V VI VII 0
Period 1
Period 2 QTWY
Period 3 RSUX
Period 4 Z
Fig. 4 — Part of the Periodic Table (columns are groups, rows are periods)

(a) State the electronic arrangement of an atom of R and of an atom of X. (2 marks)

(b) State the atomic number of Y. (1 mark)

(c) Q and Z are both in Group I. Give the electronic arrangement of an atom of Z, and explain why Q and Z are expected to have similar chemical properties. (2 marks)

(d) R, S, U and X are all in Period 3. Describe how the number of electrons in the outermost shell, and the number of occupied electron shells, change across Period 3 from R to X. (2 marks)

(e) Element M has atomic number 14. By deducing its electronic arrangement, state the group and period of M. Which element in Fig. 4 is expected to have chemical properties most similar to those of M? (2 marks)

(f) Atoms of W can gain electrons to form an ion which has the same electronic arrangement as an atom of Y. Deduce the charge on this ion. (1 mark)

10 marks
06

Section B — Comprehensive Structured Question 5

Identifying Elements from Isotope and Electronic Arrangement Data

Atoms of element M have two occupied electron shells, with three electrons in the outermost shell. Natural M consists of two isotopes: atoms of isotope M-I contain 5 neutrons, and atoms of isotope M-II contain 6 neutrons. The relative atomic mass of M is 10.8. (M is not a chemical symbol.) For the calculations in this question, take the relative isotopic mass of each isotope to be equal to its mass number.

(a) Deduce the atomic number of M, and state the group and period of M. (2 marks)

(b) State the mass number of M-I and the mass number of M-II. (1 mark)

(c) Calculate the percentage abundance of M-II in natural M. (3 marks)

(d) Explain why M-I and M-II have the same chemical properties but not identical physical properties. (2 marks)

(e) Element N is in the same group as M, and an atom of N has one more occupied electron shell than an atom of M. N has only one naturally occurring isotope, whose atoms contain 14 neutrons. Deduce the electronic arrangement of an atom of N, and calculate the relative atomic mass of N. (2 marks)

10 marks

Question 1 — Multiple Choice

1. C
2. B
3. A
4. B
5. D
6. B
7. D
8. C
9. C
10. C
11. A
12. D
13. A
14. B
15. C
16. A
17. D
18. D
19. A
20. B
  1. C — 15 protons, 16 neutrons, 18 electrons. The atomic number is 15, so there are 15 protons. Neutrons = 31 − 15 = 16. The charge of 3− means 3 extra electrons: 15 + 3 = 18. (B uses the mass number as the number of neutrons. D swaps protons and electrons.)
  2. B — W and Y. Protons = mass number − neutrons: W has 12, X has 13, Y has 12 and Z has 14 − … i.e. 27 − 14 = 13. W and Y have the same number of protons (12) but different mass numbers (24 and 25), so they are isotopes. (X and Z are also isotopes of each other, but that pair is not offered.)
  3. A — 2713Al3+ and 199F−. Al3+ has 13 − 3 = 10 electrons and F− has 9 + 1 = 10 electrons. (B: 7 and 10. C: 18 and 10. D: 16 and 18.)
  4. B — (1) and (3) only. (1) is correct: both are chlorine, with 17 protons. (2) is wrong: 3517Cl− has 35 − 17 = 18 neutrons, but 3717Cl has 37 − 17 = 20 neutrons. (3) is correct: both ions have 18 electrons, arranged 2, 8, 8.
  5. D — 2010Ne. Fluorine-19 has 19 − 9 = 10 neutrons. Neon-20 has 20 − 10 = 10 neutrons. (O-16 has 8, Na-23 has 12 and Al-27 has 14.)
  6. B — 20.0. Relative isotopic mass = mass of one atom of the isotope1/12 × mass of one C-12 atom = 3.321 × 10−23(1.993 × 10−23) ÷ 12 = 20.0. (1.67 is the ratio to a whole C-12 atom, without multiplying by 12.)
  7. D — 28.1. (28.0 × 92.2 + 29.0 × 4.7 + 30.0 × 3.1) ÷ 100 = (2581.6 + 136.3 + 93.0) ÷ 100 = 28.109 ≈ 28.1. (29.0 is the unweighted mean of the three isotopes.)
  8. C — 69.8. (69.0 × 3 + 71.0 × 2) ÷ 5 = 349 ÷ 5 = 69.8. (70.0 is the unweighted mean of 69.0 and 71.0.)
  9. C — 35%. Let the abundance of E-54 be x%. Then E-50 is (100 − 40 − x) = (60 − x)%. 50.0(60 − x) + 52.0 × 40 + 54.0x = 52.2 × 100, so 3000 − 50x + 2080 + 54x = 5220, 4x = 140 and x = 35. (E-50 is 25%.)
  10. C — (2) and (3) only. (1) is wrong: the relative atomic mass is a weighted average, so it is usually not a whole number (e.g. 35.5 for chlorine). (2) and (3) are correct.
  11. A — Atomic number 16, Group VI, Period 3. The electronic arrangement is 2, 8, 6, so the neutral atom has 16 electrons and 16 protons. There are 3 occupied shells (Period 3) and 6 outermost electrons (Group VI). The element is sulphur.
  12. D — 2, 8, 8, 2. J2+ has 18 electrons, so a J atom has 18 + 2 = 20 electrons: 2, 8, 8, 2. (A comes from subtracting the charge instead of adding it.)
  13. A — Y is in Group V and Period 3. Y3− has 18 electrons, so a Y atom has 15 electrons (and 15 protons), arranged 2, 8, 5. This gives Group V, Period 3. The mass number is 15 + 16 = 31, not 34.
  14. B — 12. The first shell holds 2 electrons, so the outermost shell holds 2 electrons. In Period 3 the arrangement is 2, 8, 2, giving 12 electrons. (4 has 2, 2 but is in Period 2. 20 has 2, 8, 8, 2 and is in Period 4.)
  15. C — 11. X is in Group V: 5 outermost electrons. Y is in Group IV: 4 outermost electrons. Z is helium (Period 1, Group 0), which has only 2 electrons, all in its first shell. Total = 5 + 4 + 2 = 11. (9 treats Z as 0. 17 treats Z as 8.)
  16. A — T−. L (Period 3, Group 0) is argon, 2, 8, 8. T (Period 3, Group VII) has 17 electrons, so T− has 18 electrons: 2, 8, 8. (U− and X3− both have 10 electrons: 2, 8. R2+ has 2 electrons.)
  17. D — R and S. R and S are both in Group II and have 2 outermost electrons. (A and B give wrong reasons. Similar properties depend on the number of outermost electrons, not on the period or the number of shells. In C, Z has 2 outermost electrons but L has 8.)
  18. D — (1), (2) and (3). (1) The period number equals the number of occupied shells. (2) Na to Ar have 1, 2, 3, 4, 5, 6, 7, 8 outermost electrons. (3) Period 2 runs from lithium to neon: eight elements.
  19. A — (1) and (2) only. F is 2, 7 and Cl is 2, 8, 7. Both have 7 outermost electrons, and Cl has 3 shells against 2. (3) is wrong because the number of outermost electrons is the same within a group.
  20. B — Group I, Period 4. K+ has 18 electrons, so the K atom has 19 electrons (and 19 protons): 2, 8, 8, 1. There are 4 shells (Period 4) and 1 outermost electron (Group I). The element is potassium.

Question 2 — Atoms, Ions and Isotopes

(a) Isotopes

R and T (1)
Both have 10 protons (the same atomic number), so they are atoms of the same element (neon), but they have different numbers of neutrons (10 and 12). (1)

(b) Nuclide symbols

P: 8 protons is oxygen. Mass number = 8 + 8 = 16. Charge = 8 − 10 = −2.
P = 168O2− (1)
U: 12 protons is magnesium. Mass number = 12 + 14 = 26. Charge = 12 − 10 = +2.
U = 2612Mg2+ (1)

(c) Electronic arrangement

(i) Each particle has 10 electrons: 2, 8 (1)
(ii) A neutral atom has equal numbers of protons and electrons: R and T (1)

(d) The student’s statement

The statement is wrong. (1)
Isotopes are atoms of the same element, i.e. with the same number of protons. P has 8 protons and Q has 9 protons, so they are particles of different elements (oxygen and fluorine). Having the same number of electrons does not make particles isotopes. (1)

(e) The ion W3+

(i) Protons = 27 − 14 = 13. Electrons = 13 − 3 = 10. (1)
(ii) A neutral W atom has 13 electrons: 2, 8, 3 (1)
Common errors: (1) calling two particles isotopes because they have the same number of electrons; (2) finding the charge as electrons − protons instead of protons − electrons; (3) using the number of electrons rather than the number of protons to decide which element a particle belongs to.

Question 3 — Relative Isotopic Mass and Relative Atomic Mass of Neon

(a) Relative isotopic mass

The mass of an atom of the isotope relative to 1/12 of the mass of an atom of carbon-12. (1)

(b) Mass of one neon-22 atom

Mass = 22.0 × 1.993 × 10−2312 = 22.0 × 1.661 × 10−24 = 3.65 × 10−23 g (1)

(c) Relative atomic mass of neon

Relative atomic mass = sum of (relative isotopic mass × fractional abundance) (1)
= 20.0 × 90.5 + 21.0 × 0.3 + 22.0 × 9.2100 = 1810 + 6.3 + 202.4100 (1)
= 20.187 = 20.2 (1)

(d) Particles in neon-22

10 protons, 12 neutrons, 10 electrons (1)

(e) An enriched sample

(i) 20.0 × 0.800 + 22.0 × 0.200 = 16.0 + 4.4 = 20.4 (1)
(ii) The relative atomic mass is a weighted average, so it depends on the percentage abundances of the isotopes. This sample contains a larger proportion of the heavier neon-22 atoms, so its value is greater. (1)

(f) Position of neon

Electronic arrangement: 2, 8 (1)
Two occupied shells and a completely filled outermost shell: Group 0, Period 2 (1)
Common errors: (1) taking the simple average of 20, 21 and 22; (2) forgetting to divide by 100 when the abundances are given as percentages; (3) in (b), multiplying by 12 instead of dividing by 12.

Question 4 — Electronic Arrangement: Identifying Five Elements

(a) Completed table

Element Electronic arrangement Group Period
A2, 8, 2II3
B2, 8, 7VII3
C2, 2II2
D2, 8, 3III3
E2, 8, 8, 2II4
Reasoning (1 mark per correct row): A: 3 shells with 2 outermost electrons gives 2, 8, 2 (magnesium). B: B− has 18 electrons, so a B atom has 17 electrons: 2, 8, 7 (chlorine). C: let the outermost shell hold n electrons. Then the atomic number is 2 + n and n = (2 + n) ÷ 2, so n = 2, giving 2, 2 (beryllium). D: Period 3 (same as B) and Group III gives 2, 8, 3 (aluminium). E: Group II (same as A) with 4 shells gives 2, 8, 8, 2 (calcium).

(b) Electronic arrangement of B−

17+ 1−
Model answer for (b)
Nucleus labelled 17+; shells containing 2, 8 and 8 electrons (18 in total); charge 1− shown. (1)

(c) Similar chemical properties of A and C

Yes. (1)
Atoms of A (2, 8, 2) and C (2, 2) both have 2 electrons in the outermost shell (both are in Group II). Chemical properties depend on the number of outermost electrons. (1)

(d) Why helium is in Group 0

The first shell can hold only 2 electrons, so the outermost shell of a helium atom is completely filled. Neon atoms (2, 8) also have a completely filled outermost shell. (1)
Elements are grouped by their chemical properties. Helium and neon are both very unreactive (stable), unlike the Group II elements, whose atoms have an outermost shell that is not full. (1)
Common errors: (1) forgetting that an ion B− has one more electron than the atom; (2) writing the group number from the number of shells; (3) placing helium in Group II because it has 2 outermost electrons.

Question 5 — Periods and Groups of the Periodic Table

(a) Electronic arrangements of R and X

R (Group II, Period 3): 2, 8, 2 (1)
X (Group VII, Period 3): 2, 8, 7 (1)

(b) Atomic number of Y

Y (Group 0, Period 2) has the arrangement 2, 8, so the atomic number is 2 + 8 = 10. (1)

(c) Q and Z

Z (Group I, Period 4): 2, 8, 8, 1 (1)
Q (2, 1) and Z (2, 8, 8, 1) both have 1 electron in the outermost shell, so they have similar chemical properties. (1)

(d) Trends across Period 3

The number of outermost electrons increases from 2 (R) to 3 (S), 5 (U) and 7 (X), i.e. it increases by one for each step to the next group. (1)
The number of occupied electron shells stays the same (3) for all of them. (1)

(e) Element M (atomic number 14)

Electronic arrangement 2, 8, 4: Group IV, Period 3 (1)
Most similar to T, which is also in Group IV (4 outermost electrons). (1)

(f) Ion of W

W is in Group VI, Period 2: 2, 6. To reach 2, 8 it must gain 2 electrons, so the ion has a charge of 2− (1)
Common errors: (1) saying the number of outermost electrons stays the same across a period; (2) confusing the group number with the number of shells; (3) in (e), matching M with S because it is “next to” it in the same period instead of looking for the same group.

Question 6 — Identifying Elements from Isotope and Electronic Arrangement Data

(a) Atomic number, group and period of M

Two shells with 3 outermost electrons: electronic arrangement 2, 3. Atomic number = 2 + 3 = 5 (1)
Group III, Period 2 (1)

(b) Mass numbers

M-I: 5 + 5 = 10. M-II: 5 + 6 = 11. (1)

(c) Percentage abundance of M-II

Let the percentage abundance of M-II be x%. Then M-I is (100 − x)%. (1)
10.0(100 − x) + 11.0x100 = 10.8 (1)
1000 + x = 1080, so x = 80% (1)

(d) Chemical and physical properties of isotopes

Chemical properties depend on the number of electrons and their arrangement (especially the outermost electrons). M-I and M-II have the same number of electrons / same electronic arrangement. (1)
They have different numbers of neutrons, so different masses, and physical properties that depend on mass (e.g. density) differ. (1)

(e) Element N

N is in Group III and has 3 shells: 2, 8, 3, so it has 13 protons. (1)
Mass number = 13 + 14 = 27. With only one isotope, the relative atomic mass equals its relative isotopic mass: 27.0 (1)
Common errors: (1) leaving M-I and M-II as 5 and 6 instead of adding the 5 protons; (2) averaging 10 and 11 without using the abundances; (3) in (e), applying a weighted average to an element with only one isotope.
Mark weighting: Section A (20) + Questions 2 to 6 (5 × 10) = 70 marks.
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