Explain why elements in Period 3 show a trend from metallic to non-metallic character as you move from left to right across the period.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Period 3 of the periodic table contains elements ranging from sodium (Na) to argon (Ar). Chemists observe that the physical and chemical properties of these elements change systematically across the period.
Model answer (5 marks)
1. The atomic number (number of protons) rises from 11 (Na) to 18 (Ar) across Period 3.
2. The outermost shell contains 1 to 8 electrons, filling the 3s and 3p orbitals.
3. On the left, elements have few outer electrons and low ionisation energies, so they lose electrons easily and are metallic.
4. On the right, elements have many outer electrons and high electron affinity; they tend to gain or share electrons and are non‑metallic.
5. Because all elements use the same third shell, the increasing nuclear charge pulls the outer electrons more strongly, making electron loss harder and electron gain more favourable as you move right.
2. The outermost shell contains 1 to 8 electrons, filling the 3s and 3p orbitals.
3. On the left, elements have few outer electrons and low ionisation energies, so they lose electrons easily and are metallic.
4. On the right, elements have many outer electrons and high electron affinity; they tend to gain or share electrons and are non‑metallic.
5. Because all elements use the same third shell, the increasing nuclear charge pulls the outer electrons more strongly, making electron loss harder and electron gain more favourable as you move right.
Examiner tips
- Use the exact terms: atomic number, ionisation energy, electron affinity, metallic/non‑metallic.
- Show the trend in outer electrons (1→8) to explain the change in behaviour.
- Mention the same shell to justify the effect of nuclear charge.
- Keep the answer concise and point‑wise to match the 5 marks.
Common mistakes
- Confusing ionisation energy with electron affinity.
- Saying the trend is due to increasing mass instead of nuclear charge.
- Using vague phrases like "more electronegative" without linking to electron count.
Mark scheme (5 marks)
- The number of protons (atomic number / nuclear charge) increases across the period
- The number of electrons in the outer shell increases from 1 to 8 across the period
- Metals on the left lose electrons easily to form positive ions / have low ionisation energies
- Non-metals on the right tend to gain electrons or share electrons / have nearly full outer shells
- The same (third) electron shell is used across the whole period, so the increasing nuclear charge pulls outer electrons more strongly to the right, making it harder to lose electrons
Key terms in this question
period · metallic character · non-metallic character
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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