AS Level Chemistry chapter 1: Atomic structure MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

94 exam-style questions on Atomic structure, 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.

94 questions · 4 topics · 10 per test

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What chapter 1 covers

Ten Atomic structure questions with answers

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

1. Which equation represents the third ionisation energy of aluminium?

  1. A
    Al2+(g) → Al3+(g) + e−
  2. B
    Al(g) → Al3+(g) + 3e−
  3. C
    Al3+(g) → Al4+(g) + e−
  4. D
    Al2+(aq) → Al3+(aq) + e−
Show answer

Answer: A. The third ionisation energy removes one electron from each gaseous 2+ ion. Al(g) → Al3+(g) + 3e− is the sum of the first three; Al3+ → Al4+ is the fourth.

2. Why is the second ionisation energy of an element always greater than its first?

  1. A
    the second electron is always removed from an inner shell
  2. B
    the second electron is removed from a positive ion, in which the same nuclear charge attracts fewer electrons
  3. C
    the nuclear charge increases after the first electron is removed
  4. D
    the second electron is always in a higher-energy sub-shell
Show answer

Answer: B. After one electron has gone, the same number of protons holds fewer electrons, repulsion is less and the ion is smaller, so more energy is needed. The second electron is not always from an inner shell (for magnesium both come from 3s).

3. Why do carbon-12 and carbon-14 have the same chemical properties?

  1. A
    they have the same number of electrons, arranged in the same way
  2. B
    they have the same mass
  3. C
    they have the same number of neutrons
  4. D
    their nuclei are equally stable
Show answer

Answer: A. Chemical reactions involve electrons. Both isotopes have 6 protons and so 6 electrons in the same configuration, 1s2 2s2 2p2; only the number of neutrons differs.

4. What is the ground-state electronic configuration of a copper atom (proton number 29)?

  1. A
    [Ar] 3d10 4s1
  2. B
    [Ar] 3d9 4s2
  3. C
    [Ar] 3d10 4p1
  4. D
    [Ar] 3d8 4s2 4p1
Show answer

Answer: A. Copper is an exception: a full 3d sub-shell is more stable, so one 4s electron occupies the 3d, giving [Ar] 3d10 4s1 rather than the predicted [Ar] 3d9 4s2.

5. Why is a chloride ion, Cl−, larger than a chlorine atom?

  1. A
    Cl− has one more occupied electron shell than Cl
  2. B
    the nuclear charge of Cl− is smaller than that of Cl
  3. C
    the extra electron increases repulsion between the outer electrons while the nuclear charge stays the same
  4. D
    the extra electron goes into a 3d orbital, further from the nucleus
Show answer

Answer: C. The added electron goes into the same third shell (3p), so there is more electron–electron repulsion for the same +17 nuclear charge and the electron cloud spreads out. No new shell is occupied.

6. Which statement about protons, neutrons and electrons is correct?

  1. A
    a proton is about 1840 times lighter than an electron
  2. B
    a neutron has a relative charge of +1
  3. C
    a proton and an electron carry charges of equal size but opposite sign
  4. D
    an electron has a relative mass of 1
Show answer

Answer: C. The proton (+1) and electron (−1) have charges of the same size. It is the electron that is about 1840 times lighter than a proton, with relative mass about 1/1840; the neutron is uncharged.

7. Which ion has the same number of electrons as an argon atom (proton number 18) but more protons? (Proton numbers: Na 11, S 16, Cl 17, K 19.)

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

Answer: C. K+ has 19 protons and 19 − 1 = 18 electrons. S2− and Cl− also have 18 electrons but fewer protons than argon; Na+ has only 10 electrons.

8. Separate beams of protons, neutrons and electrons, all moving at the same velocity, pass through the same electric field. Which statement is correct?

  1. A
    electrons are deflected more than protons, and in the opposite direction
  2. B
    protons are deflected more than electrons because they carry a positive charge
  3. C
    neutrons are deflected towards the positive plate
  4. D
    protons and electrons are deflected equally but in opposite directions
Show answer

Answer: A. Deflection depends on charge divided by mass. Protons and electrons have equal-sized but opposite charges, and the electron has only about 1/1840 of the mass, so it is deflected far more. Neutrons are uncharged and pass straight through.

9. Which species is a free radical?

  1. A
    Cl−
  2. B
    Cl2
  3. C
    CH3+
  4. D
    •CH3
Show answer

Answer: D. A free radical has one or more unpaired electrons. The methyl radical •CH3 has seven electrons around carbon, one of them unpaired. Ions and Cl2 have all electrons paired.

10. Which list places the ions in order of increasing radius?

  1. A
    Na+ < Mg2+ < Al3+ < Cl−
  2. B
    Al3+ < Mg2+ < Na+ < Cl−
  3. C
    Cl− < Na+ < Mg2+ < Al3+
  4. D
    Al3+ < Mg2+ < Cl− < Na+
Show answer

Answer: B. Na+, Mg2+ and Al3+ all have two shells (10 electrons) and shrink as nuclear charge rises from +11 to +13. Cl− has three occupied shells, so it is the largest.

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How many Atomic structure MCQs are there?

94, each with the answer and a short explanation, across 4 topics: Particles in the atom and atomic radius, Isotopes, Electrons, energy levels and atomic orbitals, Ionisation energy. Every chapter test draws ten of them, so you can take it many times.

Is the AS Level Chemistry chapter 1 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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