Describe how the properties of elements change across Period 3 of the periodic table, from sodium to argon. Include at least one physical property and one chemical property in your answer.

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

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

Period 3 contains the elements sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), and argon (Ar).

Model answer (5 marks)

Across Period 3 the elements change from metals on the left to non‑metals on the right.
1. Metallic character decreases: Na, Mg and Al are metals; P, S and Cl are non‑metals.
2. Physical property – melting point: increases from Na to Si (Si has the highest MP) then falls sharply to Ar.
3. Physical property – Argon is a monatomic gas with very low MP and BP.
4. Chemical property – reactivity of metals falls from Na to Al: Na reacts vigorously with water, Al does not react noticeably.
5. Chemical property – reactivity of non‑metals rises from S to Cl: Cl is more reactive than S or P.

Examiner tips

  • Use the word "decreases" or "increases" to show direction of change. Mention specific elements (Na, Mg, Al, Si, P, S, Cl, Ar) to show breadth. Include both a physical and a chemical property as required.
  • common_mistakes
  • :
  • Mixing up the order of elements or forgetting that Ar is a noble gas. Using a property that is not mentioned in the mark scheme (e.g. density) without justification. Not distinguishing between metallic and non‑metallic reactivity.

Mark scheme (5 marks)

  1. Metallic character decreases across Period 3 (from metals on the left to non-metals on the right) / sodium, magnesium and aluminium are metals; phosphorus, sulfur and chlorine are non-metals
  2. Melting point generally increases from sodium to silicon (silicon has the highest melting point) then decreases from silicon to argon
  3. Argon (noble gas) has very low / negligible melting and boiling points and exists as a monatomic gas at room temperature
  4. Reactivity of metals decreases from sodium to aluminium (sodium is most reactive metal in the period) / sodium reacts vigorously with water but aluminium does not react noticeably with water
  5. Reactivity of non-metals increases from sulfur to chlorine across the period / chlorine is more reactive than sulfur or phosphorus / non-metal reactivity increases going right (towards Group 7)

Key terms in this question

Period 3

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