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HKDSE Chemistry Recap: Microscopic World I | Vanilla (2026)

HKDSE Chemistry — Microscopic World I

Subject: Chemistry

Vanilla (2026)

Issue date: Friday, 2 October 2026

01

Section A — Atomic Structure, Periodic Table and Group Trends

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. In a neutral atom, the number of neutrons must equal the number of protons.
  2. An atom of 3717Cl contains three more neutrons than an atom of 3517Cl.
  3. The nucleus of an atom occupies only a very small part of the atom but contains nearly all its mass.
  4. An electron and a proton have equal relative masses but opposite relative charges.
  5. The particles 4020Ca2+ and 4018Ar have the same numbers of protons, neutrons and electrons.
  6. Isotopes of the same element must occupy the same position in the Periodic Table.
  7. Two atoms having the same mass number must be isotopes of the same element.
  8. The relative atomic mass of a naturally occurring element may change slightly if the relative abundances of its isotopes in the sample are different.
  9. If element X consists only of isotopes 10X and 11X, its relative atomic mass cannot be 11.2.
  10. An element with the electronic arrangement 2,8,8,2 is in Period 4 and Group II.
  11. Two elements in the same period must have the same number of outermost-shell electrons.
  12. An element with atomic number 9 and an element with atomic number 17 show similar chemical properties.
  13. A Group VII atom normally gains one electron when it reacts because this gives it a stable outermost-shell electronic arrangement.
  14. The reactivity of Group VII elements increases down the group because their atoms have progressively more outermost-shell electrons.
  15. Chlorine is more reactive than bromine because the outermost shell of a chlorine atom is closer to its nucleus and experiences less electron shielding.
  16. A bromine atom has a greater nuclear charge than a chlorine atom; nevertheless, it has a weaker attraction for an incoming electron.
  17. All Group 0 atoms have eight electrons in their outermost shells.
  18. Group 0 elements are generally chemically stable because their atoms already have stable outermost-shell electronic arrangements.
  19. Potassium reacts more vigorously with water than sodium because the outermost electron of a potassium atom is farther from and less strongly attracted to its nucleus.
  20. A Period 3 Group II metal is generally less reactive than the Period 3 Group I metal because its outermost electrons are more strongly attracted to a nucleus with a greater nuclear charge.
20 marks
02

Section B — Comprehensive Structured Question 1

Atomic Structure and an Unusual Isotopic Sample

Element X has atomic number 17. A natural sample of X contains three isotopes: 35X, 36X and 37X. Their relative isotopic masses are 34.97, 35.97 and 36.97 respectively.

The percentage abundance of 36X is one-fifth of the percentage abundance of 35X. The relative atomic mass of X in the sample is 35.45.

(a) Atomic structure

For an atom of 37X, state the number of:

  1. protons; (1 mark)
  2. neutrons; (1 mark)
  3. electrons. (1 mark)

(b) Isotopes

Explain why 35X, 36X and 37X are isotopes of the same element. (2 marks)

(c) Isotopic abundances

Let the fractional abundance of 35X be p.

  1. Write, in terms of p, the fractional abundances of 36X and 37X. (2 marks)
  2. Using the relative atomic mass of the sample, determine the percentage abundance of each isotope. (3 marks)
10 marks
03

Section B — Comprehensive Structured Question 2

Unfamiliar Elements in the Periodic Table

The letters P, Q, R, S and T do not represent the actual symbols of the elements.

Element Atomic number
P2
Q9
R11
S17
T18

(a) Electronic arrangements

Write the electronic arrangement of each of the following:

  1. Q; (1 mark)
  2. R; (1 mark)
  3. T. (1 mark)

(b) Position in the Periodic Table

State the period and group of S. (2 marks)

(c) Group 0 elements

Both P and T are Group 0 elements. Explain why their outermost-shell electronic arrangements are different, but both elements are chemically stable. (2 marks)

(d) Group VII elements

Q and S are both Group VII elements.

  1. State which one is more reactive. (1 mark)
  2. Explain the difference in their reactivity in terms of: the number of occupied electron shells; electron shielding; and the attraction for an incoming electron. (2 marks)
10 marks
04

Section B — Comprehensive Structured Question 3

Group I and Group II: Evidence from Reactions

Four metals W, X, Y and Z are known to be lithium, sodium, magnesium and calcium, but not necessarily in that order. The following experiment is conducted.

Experiment 1: Reaction with cold water

Metal Observation
W Floats and moves slowly on the water surface; bubbles are produced
X Floats, melts into a silvery ball and moves rapidly; vigorous bubbling occurs
Y No obvious reaction within a short period
Z Sinks; steady bubbling occurs; the mixture gradually becomes cloudy

(a) Identifying the metals

Identify W, X, Y and Z. (4 marks)

(b) Reactivity down Group I

Using electronic arrangements, explain why X is more reactive than W. Your answer should refer to:

  • the distance of the outermost electron from the nucleus;
  • electron shielding;
  • the attraction between the nucleus and the outermost electron.

(3 marks)

(c) Reactivity across Period 3

X and Y are both Period 3 metals. Explain why X is more reactive than Y. (2 marks)

(d) Evaluating a conclusion

A student says: “Metal Z is less reactive than X because Z sinks in water while X floats.” Explain why this conclusion is invalid. (1 mark)

10 marks

Question 1 — True or False

  1. False. A neutral atom has equal numbers of protons and electrons. Its number of neutrons may be different.
  2. False. 3717Cl has two more neutrons than 3517Cl (20 neutrons compared with 18).
  3. True.
  4. False. An electron and a proton have opposite relative charges, but a proton has a much greater relative mass than an electron.
  5. False. 4020Ca2+ (20 p, 20 n, 18 e) and 4018Ar (18 p, 22 n, 18 e) have the same number of electrons but different numbers of protons and neutrons.
  6. True. Isotopes of the same element have the same atomic number.
  7. False. Isotopes must have the same atomic number but different mass numbers. Atoms of different elements can have the same mass number.
  8. True.
  9. True. The relative atomic mass is a weighted average, so it must lie between 10 and 11.
  10. True. 2,8,8,2 has four occupied shells and two outermost-shell electrons.
  11. False. Elements in the same period have the same number of occupied electron shells. Their numbers of outermost-shell electrons are different.
  12. True. Both are Group VII elements with seven outermost-shell electrons (2,7 and 2,8,7).
  13. True.
  14. False. All Group VII atoms have seven outermost-shell electrons. The reactivity of Group VII elements decreases down the group.
  15. True.
  16. True. Bromine has more occupied shells and greater electron shielding, so its attraction for an incoming electron is weaker.
  17. False. Helium has two electrons in its only occupied electron shell. The other Group 0 atoms have eight outermost-shell electrons.
  18. True.
  19. True.
  20. True.
Group trends to remember: the reactivity of Group I and Group II metals increases down the group, while the reactivity of Group VII non-metals decreases down the group. Group 0 elements are generally chemically stable.

Question 2 — Atomic Structure and an Unusual Isotopic Sample

(a) Atomic structure

For 3717X:

  1. Protons: 17 (1)
  2. Neutrons: 37 − 17 = 20 (1)
  3. Electrons: 17 (1)

(b) Isotopes

  • They have the same atomic number, and hence the same number of protons. (1)
  • They have different mass numbers, and hence different numbers of neutrons. (1)

(c) Isotopic abundances

  1. Fractional abundance of 35X = p
    Fractional abundance of 36X = p ÷ 5 (1)
    Fractional abundance of 37X = 1 − p − p/5 = 1 − 6p/5 (1)
  2. 34.97p + 35.97(p/5) + 36.97(1 − 6p/5) = 35.45 (1)
    34.97p + 7.194p + 36.97 − 44.364p = 35.45
    36.97 − 2.20p = 35.45
    2.20p = 1.52
    p = 0.6909 (1)
    35X = 69.1%
    36X = 69.1 ÷ 5 = 13.8%
    37X = 100 − 69.1 − 13.8 = 17.1% (1)
Check: 0.6909 × 34.97 + 0.1382 × 35.97 + 0.1709 × 36.97 ≈ 35.45, which matches the given relative atomic mass.

Question 3 — Unfamiliar Elements in the Periodic Table

(a) Electronic arrangements

  1. Q (atomic number 9): 2,7 (1)
  2. R (atomic number 11): 2,8,1 (1)
  3. T (atomic number 18): 2,8,8 (1)

(b) Position of S

S has atomic number 17, so its electronic arrangement is 2,8,7.

  • Period: 3 (three occupied electron shells) (1)
  • Group: VII (seven outermost-shell electrons) (1)

(c) Group 0 elements

  • P has the electronic arrangement 2. Its first and only electron shell is completely filled. T has the electronic arrangement 2,8,8, with eight electrons in its outermost shell. (1)
  • Both therefore have stable outermost-shell electronic arrangements and do not readily gain or lose electrons. (1)

(d) Group VII elements

  1. Q is more reactive. (1)
  2. Compared with Q, an atom of S:
    • has more occupied electron shells;
    • has greater electron shielding;
    • has its outermost shell farther from the nucleus;
    • therefore has a weaker attraction for an incoming electron.
    Thus, S gains an electron less readily and is less reactive than Q. (2)
Marking note: In (d)(2), award 1 mark for more occupied shells and greater shielding in S, and 1 mark for the outermost shell being farther from the nucleus so that the attraction for an incoming electron is weaker.

Question 4 — Group I and Group II: Evidence from Reactions

(a) Identifying the metals

W = lithium (1)
X = sodium (1)
Y = magnesium (1)
Z = calcium (1)

(b) Sodium compared with lithium

Lithium has the electronic arrangement 2,1, while sodium has the electronic arrangement 2,8,1. In sodium:

  • the outermost electron is farther from the nucleus; (1)
  • there are more inner-shell electrons, so electron shielding is greater; (1)
  • the attraction between the nucleus and the outermost electron is weaker, so the outermost electron is lost more readily. (1)

Hence, sodium is more reactive than lithium.

(c) Sodium compared with magnesium

Sodium has the electronic arrangement 2,8,1, while magnesium has the electronic arrangement 2,8,2.

  • Magnesium has a greater nuclear charge than sodium. Their outermost electrons are in the same electron shell and experience similar shielding. (1)
  • Magnesium’s outermost electrons are therefore more strongly attracted to its nucleus. Sodium loses its single outermost electron more readily and is more reactive. (1)

(d) Evaluating the student’s conclusion

Whether a metal floats or sinks mainly depends on its density relative to water, not directly on its chemical reactivity. Reactivity should be compared using evidence such as the rate of bubbling, movement, heat produced and how quickly the metal disappears. (1)

Common error: Do not use floating or sinking as evidence of reactivity. Compare the rate of bubbling and other signs of reaction instead.
Mark weighting: Section A (20) + Question 2 (10) + Question 3 (10) + Question 4 (10) = 50 marks.
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