Explain why the elements in Period 3 show a gradual transition from metallic to non-metallic character across the period, from sodium (Na) to chlorine (Cl).
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Across Period 3 the nuclear charge increases from 11 p (Na) to 17 p (Cl) while the shielding from the two inner shells remains essentially unchanged. Consequently the effective nuclear charge on the valence electrons rises, pulling them closer to the nucleus and raising ionisation energies and electron affinities.
On the left of the period the elements (Na, Mg, Al) have only one or two valence electrons and low ionisation energies; they lose electrons easily and are metallic.
Towards the right (P, S, Cl) the valence shells are nearly full; the high ionisation energies and electron affinities mean they tend to gain or share electrons, giving non‑metallic character.
On the left of the period the elements (Na, Mg, Al) have only one or two valence electrons and low ionisation energies; they lose electrons easily and are metallic.
Towards the right (P, S, Cl) the valence shells are nearly full; the high ionisation energies and electron affinities mean they tend to gain or share electrons, giving non‑metallic character.
Examiner tips
- Show the increase in nuclear charge and constant shielding to explain rising effective nuclear charge. Mention ionisation energies/electron affinities to link to metallic/non‑metallic behaviour. Use specific element examples (Na, Mg, Al, P, S, Cl).
Common mistakes
- Confusing the trend of ionisation energy with electron affinity. Forgetting that shielding does not change significantly across a period. Using vague terms like "more electronegative" without linking to effective nuclear charge.
Mark scheme (4 marks)
- Nuclear charge (number of protons) increases across Period 3, so the nucleus attracts electrons more strongly.
- Shielding (from inner electrons) remains approximately constant across the period, so effective nuclear charge experienced by valence electrons increases.
- Elements on the left (e.g. Na, Mg, Al) have few valence electrons and low ionisation energies, so they readily lose electrons — characteristic of metals.
- Elements on the right (e.g. P, S, Cl) have many valence electrons and high electron affinities / ionisation energies, so they tend to gain or share electrons — characteristic of non-metals.
Key terms in this question
metallic character · non-metallic character
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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