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HKDSE Chemistry Recap: Elements, Isotopes and Relative Atomic Mass | Joe (2026)

HKDSE Chemistry — Elements, Isotopes and Relative Atomic Mass

Subject: Chemistry

Joe (2026)

Issue date: Thursday, 8 October 2026

Total: 50 marks  |  Suggested time: 60–70 minutes  |  A Periodic Table is required. Relative isotopic masses are given where needed. Answers to calculations should be given to 3 significant figures unless stated otherwise.

01

Section A — Elements, Atoms and the Periodic Table

Multiple Choice

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

  1. Fig. 1 shows four particle diagrams. In each diagram, ○ and ● represent atoms of two different elements, and atoms that touch are joined together. Which diagram represents a compound?
    A B C D
    Fig. 1 — ○ and ● are atoms of two different elements
    1. Diagram A
    2. Diagram B
    3. Diagram C
    4. Diagram D
  2. Which of the following statements about compounds and mixtures is correct?
    1. The components of a mixture are chemically combined together.
    2. The constituents of a compound can be separated by physical methods.
    3. A compound has a fixed composition by mass, but the composition of a mixture can vary.
    4. A mixture has a fixed melting point.
  3. A particle contains 17 protons, 18 neutrons and 18 electrons. Which of the following represents this particle?
    1. 3517Cl
    2. 3617Cl−
    3. 3517Cl+
    4. 3517Cl−
  4. The table shows the numbers of subatomic particles in four particles W, X, Y and Z. (W, X, Y and Z are not chemical symbols.)
    Particle Protons Neutrons Electrons
    W666
    X686
    Y777
    Z8810
    Which pair of particles are isotopes of the same element?
    1. W and X
    2. X and Y
    3. W and Z
    4. Y and Z
  5. Chlorine-35 and chlorine-37 are isotopes of chlorine. Which of the following statements about these two isotopes is correct?
    1. They have different numbers of electrons.
    2. They have different chemical properties because their masses are different.
    3. They have the same chemical properties because they have the same electronic arrangement.
    4. They have the same density because they are atoms of the same element.
  6. Naturally occurring copper consists of two isotopes: copper-63 (relative isotopic mass 63.0, abundance 69.2%) and copper-65 (relative isotopic mass 65.0, abundance 30.8%). What is the relative atomic mass of copper?
    1. 63.0
    2. 63.6
    3. 64.0
    4. 64.4
  7. Boron consists of two isotopes, boron-10 (relative isotopic mass 10.0) and boron-11 (relative isotopic mass 11.0). The relative atomic mass of boron is 10.8. What is the percentage abundance of boron-11?
    1. 20%
    2. 40%
    3. 60%
    4. 80%
  8. Which of the following statements about relative atomic mass is correct?
    1. It is the weighted average of the relative isotopic masses of the naturally occurring isotopes of the element.
    2. It is always a whole number.
    3. It is the mass of one atom of the element in grams.
    4. It is the mass of one atom of the element relative to the mass of one hydrogen atom.
  9. An atom of element X has 15 protons. In which group and period of the Periodic Table is X found?
    1. Group III, Period 5
    2. Group V, Period 2
    3. Group V, Period 3
    4. Group VI, Period 3
  10. Which pair of elements, given by their atomic numbers, would be expected to have similar chemical properties?
    1. 11 and 12
    2. 9 and 17
    3. 10 and 11
    4. 6 and 8
1
2
3
4
5
6
7
8
9
10
10 marks
02

Section B — Elements, Atoms 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 element is a substance which cannot be broken down into simpler substances by chemical methods.
  2. Oxygen gas (O2) is a compound because each of its molecules contains two atoms.
  3. Isotopes of an element have the same number of neutrons but different numbers of protons.
  4. An atom is electrically neutral because it contains equal numbers of protons and electrons.
  5. Carbon-14 and nitrogen-14 are isotopes of each other because they have the same mass number.
  6. Isotopes of the same element have the same chemical properties but some different physical properties.
  7. The relative atomic mass of an element is always equal to the mass number of its most abundant isotope.
  8. Relative isotopic mass and relative atomic mass have no units.
  9. Elements in the same period of the Periodic Table have the same number of outermost-shell electrons.
  10. Silicon is a metalloid: it has some properties of metals and some properties of non-metals.
10 marks
03

Section C — Comprehensive Structured Question 1

Elements, Compounds and Mixtures

Fig. 2 shows the particles in four containers I, II, III and IV. In each container, ○ represents an atom of element X and ● represents an atom of element Y. Atoms that touch are joined together to form a molecule.

I II III IV
Fig. 2 — Particle diagrams of the contents of containers I to IV (○ = atom of X, ● = atom of Y)

(a) For each of containers I, II, III and IV, state whether it contains an element, a compound or a mixture. (2 marks)

(b) In the box below, draw a particle diagram of a pure compound formed from X and Y, in which each molecule consists of one ● atom joined to two ○ atoms. Draw at least four molecules. (1 mark)

(c) A mixture of iron powder and sulphur powder is heated strongly. The two elements combine to form a black solid, iron(II) sulphide. State two differences between the mixture of iron and sulphur and the compound iron(II) sulphide. (2 marks)

(d) Complete the table by stating whether each substance is an element, a compound or a mixture. (2 marks)

Substance Element, compound or mixture?
Brass
Ozone, O3
Calcium carbonate
Sea water

(e) When mercury(II) oxide, a red solid, is strongly heated, it forms mercury and oxygen. Explain why this shows that mercury(II) oxide is not an element. (1 mark)

(f) A student says, “A substance whose molecules each contain two atoms joined together must be a compound.” Use Fig. 2 to show that the student is wrong. (2 marks)

10 marks
04

Section C — Comprehensive Structured Question 2

Atoms, Ions and Isotopes

The table shows the numbers of subatomic particles in five particles W, X, Y, Z and V. Each particle is a single atom or a single monatomic ion. (W, X, Y, Z and V are not chemical symbols.) Use the Periodic Table where necessary.

Particle Number of protons Number of neutrons Number of electrons
W666
X686
Y111210
Z171818
V121210

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

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

(c) State the electronic arrangement of an atom of X. (1 mark)

(d) Particles Y and V have the same electronic arrangement. State this electronic arrangement, and explain why Y and V are nevertheless different species. (2 marks)

(e) Hydrogen has three isotopes: hydrogen-1 (11H), hydrogen-2 (21H) and hydrogen-3 (31H). Complete the table for neutral atoms of these isotopes. (1 mark)

Isotope Number of protons Number of neutrons Number of electrons
Hydrogen-1
Hydrogen-2
Hydrogen-3

(f) Heavy water (D2O), made from hydrogen-2, has a higher density than ordinary water (H2O), but the two behave in almost the same way in chemical reactions.

  1. Explain why isotopes of an element have the same chemical properties. (1 mark)
  2. Explain why isotopes of an element can have different physical properties. (1 mark)
10 marks
05

Section C — Comprehensive Structured Question 3

Relative Atomic Mass and Classification of Elements

Fig. 3 shows the relative abundances of the three naturally occurring isotopes of magnesium (atomic number 12), as measured by a mass spectrometer. Take the relative isotopic masses of the isotopes to be 24.0, 25.0 and 26.0.

0 20 40 60 80 79.0% 10.0% 11.0% 24 25 26 Relative isotopic mass Relative abundance (%)
Fig. 3 — Isotopes of magnesium

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

(b) Using the data in Fig. 3, calculate the relative atomic mass of magnesium. Give your answer to 3 significant figures. (3 marks)

(c) Explain why the relative atomic mass of magnesium is not a whole number, although each relative isotopic mass is a whole number (to the precision given). (1 mark)

(d) Element Y has two isotopes, Y-69 (relative isotopic mass 69.0) and Y-71 (relative isotopic mass 71.0). The relative atomic mass of Y is 69.8. Calculate the percentage abundance of Y-71. (2 marks)

(e) Magnesium is a metal in Period 3 of the Periodic Table. Chlorine (atomic number 17) is also in Period 3.

  1. State the electronic arrangement of a magnesium atom. (1 mark)
  2. State two physical properties that magnesium has because it is a metal. (1 mark)
  3. State the group and period of chlorine in the Periodic Table, and whether chlorine is a metal or a non-metal. (1 mark)
10 marks

Question 1 — Multiple Choice

1. B
2. C
3. D
4. A
5. C
6. B
7. D
8. A
9. C
10. B
  1. B — Diagram B. A compound consists of two or more different kinds of atoms chemically combined in a fixed ratio. In B, all the particles are identical molecules, each made of one ○ atom and one ● atom. (A is a mixture of two monatomic elements. C is a diatomic element. D is a mixture of an element and a compound.)
  2. C — A compound has a fixed composition by mass, but the composition of a mixture can vary. (A: the components of a mixture are not chemically combined. B: the constituents of a compound can only be separated by chemical methods. D: a mixture melts over a range of temperatures, not at a fixed melting point.)
  3. D — 3517Cl−. 17 protons means the element is chlorine. Mass number = 17 + 18 = 35. There are 18 electrons, one more than the 17 protons, so the charge is 1−.
  4. A — W and X. Isotopes are atoms of the same element, so they have the same number of protons (6) but different numbers of neutrons (6 and 8). (X and Y both have mass number 14 but different numbers of protons, so they are atoms of different elements. Z is an oxide ion, which has 8 protons.)
  5. C — They have the same chemical properties because they have the same electronic arrangement. Chemical properties depend on the electrons (especially the outermost electrons), and isotopes have the same number of electrons. Their different masses give slightly different physical properties, such as density.
  6. B — 63.6. 63.0 × 0.692 + 65.0 × 0.308 = 43.596 + 20.020 = 63.6. (64.0 is the simple average of 63 and 65, which ignores the abundances. 64.4 comes from swapping the two abundances.)
  7. D — 80%. Let the fraction of boron-11 be x: 10.0(1 − x) + 11.0x = 10.8, so 10.0 + x = 10.8 and x = 0.80, i.e. 80%. (20% is the abundance of boron-10.)
  8. A — It is the weighted average of the relative isotopic masses of the naturally occurring isotopes. It is measured relative to 1/12 of the mass of a carbon-12 atom, not to hydrogen, it has no unit, and it is usually not a whole number.
  9. C — Group V, Period 3. 15 protons means 15 electrons in the atom, arranged 2, 8, 5. There are 3 occupied shells (Period 3) and 5 outermost electrons (Group V). The element is phosphorus.
  10. B — 9 and 17. Fluorine (2, 7) and chlorine (2, 8, 7) are both in Group VII and have 7 outermost electrons, so they have similar chemical properties. (11 and 12 are neighbours in Period 3. 10 and 11 are a noble gas and an alkali metal. 6 and 8 are in the same period but different groups.)

Question 2 — True or False

  1. True.
  2. False. Oxygen gas (O2) is an element. A compound needs two or more different kinds of atoms chemically combined, and both atoms in O2 are the same.
  3. False. Isotopes of an element have the same number of protons but different numbers of neutrons.
  4. True. The positive charge of the protons is balanced by the negative charge of the electrons.
  5. False. Carbon-14 and nitrogen-14 have the same mass number but different numbers of protons (6 and 7), so they are atoms of different elements. Isotopes must be atoms of the same element.
  6. True. They have the same electronic arrangement, so the same chemical properties. Properties that depend on mass, such as density, differ slightly.
  7. False. The relative atomic mass is the weighted average of the relative isotopic masses of all the naturally occurring isotopes. It is usually not a whole number. For example, chlorine has a relative atomic mass of 35.5, although its most abundant isotope has mass number 35.
  8. True. Both are ratios of masses (relative to 1/12 of the mass of a carbon-12 atom), so the units cancel.
  9. False. Elements in the same period have the same number of occupied electron shells. Elements in the same group have the same number of outermost-shell electrons.
  10. True. Metalloids (e.g. silicon, germanium) lie between the metals and the non-metals in the Periodic Table and show properties of both.

Question 3 — Elements, Compounds and Mixtures

(a) Classifying the containers

I: element (only molecules of X2)
II: compound (only molecules of XY, each with two different atoms)
III: mixture (of the elements X2 and Y2)
IV: mixture (of the compound XY and the element X2)
(2 marks for all four correct; 1 mark for two or three correct)

(b) Particle diagram of the compound

Model answer for (b)
At least four identical molecules, each consisting of one ● atom joined to two ○ atoms, and no other kinds of particle in the box. (1)

(c) Mixture of iron and sulphur versus iron(II) sulphide

Any two of the following (1 mark each):
• Iron and sulphur in the mixture can be separated by physical methods (e.g. by a magnet, or by dissolving the sulphur in a solvent), but iron cannot be separated from iron(II) sulphide by physical methods. (1)
• The composition of the mixture can vary, but iron(II) sulphide has a fixed composition (1 : 1 ratio of Fe to S atoms). (1)
• The mixture shows the properties of iron and sulphur; iron(II) sulphide has different properties (e.g. it is a black solid). (1)
• Energy (heat) is needed to form the compound, but not to form the mixture. (1)

(d) Classifying the substances

Brass: mixture (an alloy of copper and zinc)
Ozone, O3: element
Calcium carbonate: compound
Sea water: mixture
(2 marks for all four correct; 1 mark for two or three correct)

(e) Mercury(II) oxide

It can be broken down into simpler substances (mercury and oxygen) by a chemical method (heating), so it is made of two different elements chemically combined, i.e. it is a compound. (1)

(f) A molecule with two atoms need not be a compound

In container I, each molecule contains two atoms joined together, but both atoms are ○ (the same kind), so the substance is an element (X2). (1)
A compound must contain two or more different kinds of atoms chemically combined (as in container II). (1)
Common errors: (1) calling container IV a compound because some molecules contain two kinds of atoms (it contains two different kinds of molecule, so it is a mixture); (2) calling a diatomic element such as O2 a compound; (3) in (b), drawing atoms that are not joined, or adding extra single atoms.

Question 4 — Atoms, Ions and Isotopes

(a) Isotopes

W and X (1)
They have the same number of protons (6), so they are atoms of the same element (carbon), but different numbers of neutrons (6 and 8). (1)

(b) Nuclide symbols

Y: 11 protons is sodium. Mass number = 11 + 12 = 23. Charge = 11 − 10 = +1.
Y = 2311Na+ (1)
Z: 17 protons is chlorine. Mass number = 17 + 18 = 35. Charge = 17 − 18 = −1.
Z = 3517Cl− (1)

(c) Electronic arrangement of an atom of X

X has 6 protons, so a neutral atom has 6 electrons: 2, 4 (1)

(d) Y and V

Both have 10 electrons: electronic arrangement 2, 8 (1)
They have different numbers of protons (11 and 12), so they are ions of different elements (Na+ and Mg2+). (1)

(e) Isotopes of hydrogen

Isotope Protons Neutrons Electrons
Hydrogen-1101
Hydrogen-2111
Hydrogen-3121
1 mark for all nine entries correct (neutrons = mass number − 1; protons = electrons = 1). (1)

(f) Chemical and physical properties of isotopes

(i) Chemical properties depend on the number of electrons and their arrangement (especially the outermost electrons). Isotopes of an element have the same number of electrons / same electronic arrangement. (1)
(ii) Isotopes have different numbers of neutrons, hence different masses, so physical properties that depend on mass (e.g. density, melting point, boiling point) are different. (1)
Common errors: (1) forgetting that the charge of an ion is (protons − electrons); (2) giving the mass number as protons + electrons; (3) saying that isotopes have different chemical properties because they have different masses.

Question 5 — Relative Atomic Mass and Classification of Elements

(a) Relative isotopic mass

The mass of an atom of the isotope relative to 1/12 of the mass of a carbon-12 atom (so that the relative isotopic mass of carbon-12 is exactly 12). (1)

(b) Relative atomic mass of magnesium

Relative atomic mass = sum of (relative isotopic mass × fractional abundance) (1)
= 24.0 × 79.0 + 25.0 × 10.0 + 26.0 × 11.0100 = 1896 + 250 + 286100 (1)
= 24.32 = 24.3 (1)

(c) Why it is not a whole number

The relative atomic mass is the weighted average of the relative isotopic masses of all the isotopes, taking into account their abundances, so it is generally not a whole number. (1)

(d) Abundance of Y-71

Let the percentage abundance of Y-71 be x%. Then Y-69 is (100 − x)%.
69.0(100 − x) + 71.0x100 = 69.8 (1)
6900 + 2x = 6980, so x = 40% (1)

(e) Classification of elements

(i) Magnesium has 12 protons and so 12 electrons: 2, 8, 2 (1)
(ii) Any two: good conductor of electricity; good conductor of heat; shiny (lustrous); malleable; ductile; high melting point; sonorous. (1 mark for two correct properties)
(iii) Chlorine has the electronic arrangement 2, 8, 7: Group VII, Period 3. It is a non-metal. (1)
Common errors: (1) averaging the relative isotopic masses without using the abundances; (2) forgetting to divide by 100 when abundances are given as percentages; (3) writing the group number from the number of shells instead of the number of outermost electrons.
Mark weighting: Section A (10) + Section B (10) + Question 3 (10) + Question 4 (10) + Question 5 (10) = 50 marks.
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