Explain why the atomic radius of elements decreases across Period 3 from sodium to chlorine.

AQA A-Level Chemistry (7405) — 3.2.1 Periodicity · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

All elements in Period 3 have the same number of electron shells, so the outer electrons are in the same third shell. As you move from Na to Cl the number of protons in the nucleus increases. The nuclear charge therefore increases while the shielding from inner electrons remains essentially unchanged. The greater nuclear charge pulls the outer electrons closer to the nucleus, so the atomic radius decreases across the period.

Examiner tips

Use the phrase ‘same number of electron shells’ to show understanding of shielding.
Explain that nuclear charge increases while shielding stays constant.
Show the logical chain: more protons → greater pull → smaller radius.
Use the word ‘therefore’ to link the reasoning to the conclusion.

Common mistakes

Failing to mention that the outer electrons are in the same shell.
Confusing the trend with a decrease in shielding rather than an increase in nuclear charge.
Using vague terms like ‘more attraction’ without linking to nuclear charge and shielding.

Mark scheme (5 marks)

All elements in Period 3 have the same number of electron shells / electrons are in the same outer shell (third shell)
The number of protons increases across the period
The nuclear charge / number of protons increases but the number of electron shells stays the same / shielding stays the same
The greater nuclear charge pulls the outer electrons closer to the nucleus
Therefore the atomic radius decreases (across the period)

Key terms in this question

atomic radius · Period 3