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Chemistry Pre-Course Test – Microscopic World I | Carmen (2026)

Microscopic World I
Pre-Course Test

Subject: HKDSE Chemistry

Carmen (2026)

Issue date: Tuesday, 22 September 2026

01

Atomic Structure, Periodic Table and Bonding

Multiple-Choice Questions

Choose the best answer for each question.

  1. The particles below represent atoms or ions.
    Particle Number of protons Number of neutrons Number of electrons
    P 17 18 18
    Q 18 22 18
    R 17 20 17
    S 19 20 18
    Which statement is correct?
    1. P and R are isotopes of the same element.
    2. P and Q have the same chemical properties.
    3. Q and S are isotopes of the same element.
    4. P and S are both negatively charged ions.
  2. An element consists of two isotopes with relative isotopic masses 63 and 65. Its relative atomic mass is 63.6. What is the percentage abundance of the isotope with relative isotopic mass 65?
    1. 20%
    2. 30%
    3. 40%
    4. 70%
  3. An atom of element X has the electron arrangement 2,8,6. Which statement about X is correct?
    1. X is in Period 2 and Group VI.
    2. X normally forms an ion by losing two electrons.
    3. X is in the same period as an element with electron arrangement 2,8,1.
    4. X has six occupied electron shells.
  4. Elements P, Q and R are consecutive elements in Period 3. Their numbers of outer-shell electrons are 1, 2 and 3 respectively. Which statement is correct?
    1. P, Q and R are all non-metals.
    2. Their proton numbers decrease from P to R.
    3. The oxide of P contains covalent bonds only.
    4. The attraction between the nucleus and the outer-shell electrons generally increases from P to R.
  5. Which substance contains both ionic bonds and covalent bonds?
    1. Magnesium oxide
    2. Sodium chloride
    3. Ammonium chloride
    4. Diamond
  6. Atoms X and Y have electron arrangements 2,8,2 and 2,7 respectively. What is the formula of the compound formed between X and Y?
    1. XY
    2. XY2
    3. X2Y
    4. X2Y7
  7. Which statement correctly explains why solid sodium chloride does not conduct electricity but molten sodium chloride does?
    1. The ions in solid sodium chloride have no electrical charges.
    2. Sodium chloride contains electrons that are released only when it melts.
    3. The ions are fixed in position in the solid but are mobile in the molten state.
    4. Sodium chloride molecules become ionized when the solid melts.
  8. Substances P, Q and R have the following properties:
    Substance Melting point Electrical conductivity
    P High Conducts as a solid
    Q Low Does not conduct in any state
    R High Conducts when molten but not when solid
    Which classification is correct?
    Option P Q R
    A Metal Simple molecular substance Ionic compound
    B Ionic compound Metal Giant covalent substance
    C Giant covalent substance Ionic compound Metal
    D Metal Ionic compound Simple molecular substance
    1. Classification A
    2. Classification B
    3. Classification C
    4. Classification D
  9. Both diamond and graphite consist only of carbon atoms. Which statement correctly compares their structures and properties?
    1. Both conduct electricity because each carbon atom has delocalized electrons.
    2. Diamond is soft because its carbon atoms are arranged in layers.
    3. Graphite conducts electricity because it contains delocalized electrons that can move through its structure.
    4. Graphite has a low melting point because the covalent bonds within its layers are weak.
  10. Which statement about metallic bonding is correct?
    1. A metal consists of positive ions attracted to a sea of delocalized electrons.
    2. A metal consists of negative ions surrounded by free protons.
    3. Metallic bonds are formed by the transfer of electrons between pairs of metal atoms.
    4. Metals conduct electricity only when molten because their ions cannot move in the solid state.
10 marks
02

Core Concepts

True or False

Write “True” or “False” for each statement. If a statement is false, rewrite it correctly.

  1. The mass number of an atom is equal to the total number of protons and neutrons in its nucleus.
  2. Two atoms with the same number of neutrons but different numbers of protons are isotopes of the same element.
  3. Isotopes of the same element have similar chemical properties because they have the same electron arrangement.
  4. Across a period, the proton number increases from one element to the next, while the number of occupied electron shells remains the same.
  5. An atom with the electron arrangement 2,8,8 is likely to be chemically unreactive because its outermost electron shell is complete.
  6. When magnesium reacts with chlorine, each magnesium atom gains two electrons and each chlorine atom loses one electron.
  7. A covalent bond is formed by the sharing of a pair of electrons between atoms.
  8. All substances containing covalent bonds have low melting points and boiling points.
  9. Graphite conducts electricity, but diamond does not, because graphite contains delocalized electrons that can move through its structure.
  10. Metals are malleable because layers of positive metal ions can slide past one another while the attraction between the ions and delocalized electrons is maintained.
10 marks
03

Atomic Structure and Isotopes

Structured Question

Chlorine occurs naturally as two isotopes. Information about the isotopes is shown below.

Isotope Number of protons Number of neutrons Percentage abundance
Chlorine-35 17 18 75.0%
Chlorine-37 17 20 25.0%
  1. State what is meant by the term isotope. [2 marks]
  2. For a neutral atom of chlorine-37, state:
    1. the number of protons;
    2. the number of neutrons; and
    3. the number of electrons.
    [3 marks]
  3. Calculate the relative atomic mass of chlorine using the information in the table. [3 marks]
  4. Explain why the relative atomic mass of chlorine is not a whole number. [2 marks]
  5. Write the electron arrangement of a chlorine atom. Hence, state its group and period. [3 marks]
  6. A chlorine atom forms a chloride ion, Cl−. Explain how the ion is formed and state its electron arrangement. [2 marks]
15 marks
04

Periodic Table and Ion Formation

Structured Question

Four elements, P, Q, R and S, have the following atomic structures.

Element Number of protons Electron arrangement
P 11 2,8,1
Q 12 2,8,2
R 16 2,8,6
S 17 2,8,7
  1. State the period in which all four elements are found. Explain your answer. [2 marks]
  2. State the group of each element. [4 marks]
  3. Identify each element, P, Q, R and S. [4 marks]
  4. Elements P and S react to form a compound.
    1. State the number of electrons transferred from each P atom.
    2. State the ions formed.
    3. Deduce the formula of the compound.
    [3 marks]
  5. Elements Q and S also form an ionic compound. Deduce its formula and explain how electrical neutrality is achieved. [2 marks]
  6. Which element, R or S, has the stronger attraction between its nucleus and an additional electron? Explain your answer. [1 mark]
16 marks
05

Bonding, Structure and Properties

Structured Question

The table gives information about four substances, A, B, C and D.

Substance Melting point Conductivity as a solid Conductivity when molten
A High Good Good
B High Poor Good
C Low Poor Poor
D Very high Poor Does not normally melt
  1. Substance A is a metal.
    1. Describe the structure and bonding of a metal.
    2. Explain why A conducts electricity in the solid state.
    [4 marks]
  2. Substance B is an ionic compound.
    1. Describe its giant ionic structure.
    2. Explain why it has a high melting point.
    3. Explain why it conducts electricity when molten but not when solid.
    [6 marks]
  3. Substance C is a simple molecular substance. Explain why it has a low melting point even though strong covalent bonds are present within each molecule. [3 marks]
  4. Substance D is a giant covalent substance.
    1. Suggest the identity of D if every atom forms four covalent bonds.
    2. Explain its very high melting point.
    3. Explain why it does not conduct electricity.
    [5 marks]
18 marks
06

Allotropes of Carbon

Structured Question

Diamond and graphite are allotropes of carbon.

  1. State what is meant by allotropes. [2 marks]
  2. Describe the bonding and structure in diamond. Include:
    • the number of covalent bonds formed by each carbon atom;
    • the arrangement of the atoms; and
    • the overall type of structure.
    [3 marks]
  3. Explain why diamond:
    1. has a very high melting point;
    2. is very hard; and
    3. does not conduct electricity.
    [6 marks]
  4. Describe the bonding and structure in graphite. [3 marks]
  5. Explain why graphite:
    1. conducts electricity; and
    2. is soft and slippery.
    [3 marks]
17 marks

Question 1 — Multiple-Choice Questions

  1. A. P and R have the same proton number but different numbers of neutrons.
  2. B — 30%.
    Let the abundance of isotope 65 be x.
    63(1 − x) + 65x = 63.6
    2x = 0.6
    x = 0.30 = 30%
  3. C. Both atoms have three occupied electron shells and are therefore in Period 3.
  4. D. Nuclear charge increases across the period while the number of occupied electron shells remains the same.
  5. C — Ammonium chloride. It contains covalent bonds within NH4+ and ionic attraction between NH4+ and Cl−.
  6. B — XY2. X forms X2+, while Y forms Y−.
  7. C. The ions are fixed in the solid lattice but are mobile when molten.
  8. A. P is metallic, Q is simple molecular and R is ionic.
  9. C. Graphite contains delocalized electrons that can move through its structure.
  10. A. Metallic bonding is the electrostatic attraction between positive metal ions and delocalized electrons.

Question 2 — True or False

  1. True.
  2. False. Isotopes are atoms with the same number of protons but different numbers of neutrons.
  3. True.
  4. True.
  5. True.
  6. False. A magnesium atom loses two electrons, while each chlorine atom gains one electron.
  7. True.
  8. False. Simple molecular substances generally have low melting and boiling points, but giant covalent substances have very high melting points.
  9. True.
  10. True.

Question 3 — Atomic Structure and Isotopes

  1. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
    • Protons: 17
    • Neutrons: 20
    • Electrons: 17
  3. Relative atomic mass
    = [(35 × 75.0) + (37 × 25.0)] ÷ 100
    = (2625 + 925) ÷ 100
    = 35.5
  4. The relative atomic mass is a weighted average of the isotopic masses. Chlorine contains chlorine-35 and chlorine-37 in different abundances, so the average is not a whole number.
    • Electron arrangement: 2,8,7
    • Group: Group VII
    • Period: Period 3
  6. A chlorine atom gains one electron to form Cl−. Its electron arrangement becomes 2,8,8.

Question 4 — Periodic Table and Ion Formation

  1. All four elements are in Period 3 because their atoms have three occupied electron shells.
    • P: Group I
    • Q: Group II
    • R: Group VI
    • S: Group VII
    • P: Sodium
    • Q: Magnesium
    • R: Sulfur
    • S: Chlorine
    • Each P atom loses one electron.
    • The ions formed are P+ and S−.
    • The formula is PS, corresponding to NaCl.
  5. The formula is QS2, corresponding to MgCl2. One Q2+ ion is balanced by two S− ions, giving an overall charge of zero.
  6. S has the stronger attraction. R and S have the same number of occupied electron shells, but S has a greater nuclear charge.

Question 5 — Bonding, Structure and Properties

  1. A metal has a giant structure containing positive metal ions surrounded by delocalized electrons. The attraction between the positive ions and delocalized electrons forms metallic bonds. The delocalized electrons can move through the structure and carry electric charge.
  2. B consists of a giant lattice of oppositely charged ions. Strong electrostatic attractions act between the ions in all directions. A large amount of energy is required to overcome these attractions, so B has a high melting point.

    In the solid state, the ions are fixed in position and cannot carry charge through the structure. When molten, the ions are mobile and can conduct electricity.

  3. Strong covalent bonds exist within each molecule, but melting does not break these bonds. Only the weak intermolecular forces between molecules are overcome, so little energy is required.
  4. D may be diamond. It has a giant three-dimensional covalent structure in which each carbon atom forms four strong covalent bonds. Many strong covalent bonds must be broken during melting, so a large amount of energy is required.

    Diamond does not conduct electricity because all four outer-shell electrons of each carbon atom are used in covalent bonds. It has no mobile ions or delocalized electrons.

Question 6 — Diamond and Graphite

  1. Allotropes are different structural forms of the same element in the same physical state.
  2. In diamond, each carbon atom forms four covalent bonds with four other carbon atoms. The atoms form a rigid, three-dimensional giant covalent structure.
    • High melting point: Many strong covalent bonds must be broken, requiring a large amount of energy.
    • Hardness: The carbon atoms are held in a rigid three-dimensional network by strong covalent bonds.
    • No electrical conductivity: Diamond has no mobile ions or delocalized electrons.
  4. In graphite, each carbon atom forms three covalent bonds with three other carbon atoms. The atoms form flat layers of hexagonal rings. Weak attractions act between the layers.
  5. Graphite conducts electricity because each carbon atom contributes one electron that becomes delocalized and can move through the layers.

    Graphite is soft and slippery because the weak attractions between its layers are easily overcome, allowing the layers to slide over one another.

Key comparison: Diamond and graphite both contain strong covalent bonds, but their different arrangements of carbon atoms produce very different physical properties.

Microscopic World I — Reference Summary

Isotopes: Atoms of the same element with the same proton number but different numbers of neutrons.
Period number: The number of occupied electron shells in an atom.
Group number: For the main-group elements covered, the group number is related to the number of outer-shell electrons.
Ionic bonding: The electrostatic attraction between oppositely charged ions.
Covalent bonding: The electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.
Metallic bonding: The electrostatic attraction between positive metal ions and delocalized electrons.
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