O Level / IGCSE Chemistry chapter 2: Atoms, elements and compounds MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

156 exam-style questions on Atoms, elements and compounds, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.

156 questions · 7 topics · 10 per test

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What chapter 2 covers

Ten Atoms, elements and compounds questions with answers

Try each one, then open the answer. These ten stay the same; the test draws new ones every time.

1. Why is the relative atomic mass of chlorine 35.5 and not a whole number?

  1. A
    chlorine atoms contain half a neutron
  2. B
    it is the average mass of the isotopes of chlorine, weighted by how common each one is
  3. C
    it is the simple average of chlorine-35 and chlorine-37
  4. D
    the electrons add 0.5 to the mass
Show answer

Answer: B. Ar is a weighted average: there is much more chlorine-35 than chlorine-37. A simple average would give 36.

2. Diamond is not used as an electrode. Why does diamond not conduct electricity?

  1. A
    Its covalent bonds are too weak to let electrons pass
  2. B
    Its ions are held in fixed positions in the lattice
  3. C
    All four outer electrons of each carbon atom are used in covalent bonds, so none are free to move
  4. D
    It has delocalised electrons, but they are trapped between layers
Show answer

Answer: C. In diamond every outer electron is held in one of four C–C bonds, so there are no mobile electrons. Diamond contains no ions and no layers.

3. A sample of a solid melts sharply at 80 °C. A second sample of the same solid starts to melt at 74 °C and has all melted by 78 °C. What does this show?

  1. A
    the second sample is impure, so it is a mixture
  2. B
    the second sample is a different element
  3. C
    the first sample is a mixture
  4. D
    both samples are pure, because they melt below 100 °C
Show answer

Answer: A. A pure substance melts at one sharp temperature. Melting over a lower range shows another substance is mixed in.

4. Brass, an alloy of copper and zinc, is harder and less malleable than pure copper. Why?

  1. A
    Zinc and copper form ionic bonds with each other in the alloy
  2. B
    Brass has no delocalised electrons, so its ions are held more firmly
  3. C
    The different-sized zinc atoms disrupt the regular layers, so the layers cannot slide over each other as easily
  4. D
    Brass contains a higher proportion of copper ions than pure copper
Show answer

Answer: C. Malleability depends on layers of ions sliding. Atoms of a different size distort the layers so sliding is harder. Brass is still metallic, with delocalised electrons, and still conducts.

5. In graphite, how many other carbon atoms is each carbon atom covalently bonded to, and what shape of ring do the atoms form?

  1. A
    4, hexagonal rings
  2. B
    3, pentagonal rings
  3. C
    4, square rings
  4. D
    3, hexagonal rings
Show answer

Answer: D. Each carbon in graphite bonds to three others within a flat layer of hexagonal (six-membered) rings; its fourth outer electron is delocalised. Four bonds per carbon describes diamond.

6. Which pair of ions have the same electronic configuration as each other?

  1. A
    K+ and Cl−
  2. B
    Na+ and Cl−
  3. C
    Mg2+ and S2−
  4. D
    Li+ and Na+
Show answer

Answer: A. K (2,8,8,1) loses one electron and Cl (2,8,7) gains one, so both ions are 2,8,8. Na+ and Mg2+ are 2,8 while Cl− and S2− are 2,8,8.

7. Which formula represents a molecule of an element?

  1. A
    Cl2
  2. B
    HCl
  3. C
    CO
  4. D
    CH4
Show answer

Answer: A. Cl2 contains only chlorine atoms, so it is an element even though each molecule has two atoms. The others contain two different elements, so they are compounds.

8. Mercury is a metal that is liquid at room temperature. Does liquid mercury conduct electricity, and if so, by what?

  1. A
    yes, by delocalised electrons
  2. B
    yes, by mercury ions moving through the liquid
  3. C
    no, because metals conduct only when solid
  4. D
    no, because the liquid has no free electrons
Show answer

Answer: A. Liquid mercury is still metallic: its positive ions sit in a sea of delocalised electrons, which carry the current. Unlike a molten ionic compound, the current is not carried by ions.

9. A substance cannot be broken down into simpler substances by any chemical reaction. What is it?

  1. A
    a compound
  2. B
    an element
  3. C
    a mixture
  4. D
    an alloy
Show answer

Answer: B. An element contains only one type of atom, so it cannot be split chemically into anything simpler. Compounds can be broken down into their elements.

10. Which statement about a compound is correct?

  1. A
    it contains only one type of atom
  2. B
    it can be separated into its elements by filtration
  3. C
    its elements are chemically bonded in a fixed ratio
  4. D
    its properties are the same as those of its elements
Show answer

Answer: C. A compound has two or more elements chemically combined in a fixed ratio, with properties different from its elements. A mixture can be separated physically.

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Atoms, elements and compounds: questions students ask

How many Atoms, elements and compounds MCQs are there?

156, each with the answer and a short explanation, across 7 topics: Elements, compounds and mixtures, Atomic structure and the Periodic Table, Isotopes, Ion and ionic bonds, Simple molecules and covalent bonds, Giant covalent structures, Metallic bonding. Every chapter test draws ten of them, so you can take it many times.

Is the O Level / IGCSE Chemistry chapter 2 test free?

Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.

How is the chapter test marked?

The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.

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