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AS · Practice questions · Atomic and ionic radius

Three factors, every trend.

Six original Cambridge-style questions on atomic and ionic radius: the period trend, the group trend, ions compared with their atoms, and isoelectronic series.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Name the factor every time.

01
Explain
[3 marks]

Explain why atomic radius decreases across Period 3 from sodium to argon.

  • Nuclear charge increases by one proton for each successive element. ✓
  • The added electrons enter the same outer shell (n = 3), so the distance of the outer shell from the nucleus does not increase and shielding by the inner shells stays roughly the same. ✓
  • The outer electrons therefore experience a greater attraction and are pulled closer to the nucleus, so the radius decreases. ✓
02
Explain
[3 marks]

Explain why atomic radius increases down Group 2 from beryllium to barium.

  • Each element down the group has one more occupied principal quantum shell, so the outer electrons are further from the nucleus. ✓
  • Each extra inner shell adds shielding, which reduces the attraction felt by the outer electrons. ✓
  • Although nuclear charge also increases, the extra distance and shielding outweigh it, so the radius increases. ✓
03
Explain
[2 marks]

Explain why a magnesium ion, Mg2+, is much smaller than a magnesium atom.

  • Mg loses both of its 3s electrons, so the ion's outer shell is n = 2, which is closer to the nucleus than n = 3. ✓
  • The 12 protons now attract only 10 electrons, so the remaining electrons are held more tightly and the radius decreases further. ✓
04
Explain
[2 marks]

Explain why an oxide ion, O2−, is larger than an oxygen atom.

  • The two extra electrons enter the same outer shell (n = 2) while the nuclear charge stays at 8 protons. ✓
  • More electrons means more electron-electron repulsion in the outer shell, so the electron cloud expands and the radius increases. ✓
05
Analysis
[3 marks]

The ions S2−, Cl−, K+ and Ca2+ are isoelectronic. Arrange them in order of increasing ionic radius and explain the order.

  • All four ions have 18 electrons (1s22s22p63s23p6). ✓
  • The only difference is the nuclear charge: S 16, Cl 17, K 19, Ca 20 protons. More protons attract the same 18 electrons more strongly, giving a smaller ion. ✓
  • Increasing radius: Ca2+ < K+ < Cl− < S2−. ✓
06
Analysis
[4 marks]

Typical radii: Na 0.186 nm, Na+ 0.095 nm, Cl 0.099 nm, Cl− 0.181 nm. Use the data to explain why the sodium ion is smaller than its atom and the chloride ion larger than its atom.

  • Na to Na+: the radius roughly halves because the single 3s electron is removed, leaving n = 2 as the outer shell. ✓
  • The unchanged 11 protons now attract only 10 electrons, so those are held more tightly. ✓
  • Cl to Cl−: the radius nearly doubles because an extra electron is added to the same n = 3 shell. ✓
  • Nuclear charge stays at 17, but the extra electron increases repulsion between the outer electrons, so the cloud expands. ✓

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