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AS · Practice questions · Period 3: physical properties

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Six original Cambridge-style questions on the physical properties of the Period 3 elements.

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.
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Nuclear charge up, shielding the same.

01
Explain
[2 marks]

Explain why atomic radius decreases across Period 3.

  • Nuclear charge increases from Na to Cl. ✓
  • The electrons are added to the same (third) shell, so shielding hardly changes and the outer electrons are pulled closer. ✓
02
Explain
[4 marks]

Explain the variation in melting point from sodium to argon.

  • Na, Mg, Al: giant metallic; melting point rises as the number of delocalised electrons and the ionic charge increase and the ions get smaller. ✓
  • Si: giant covalent, so many strong covalent bonds must be broken; highest melting point. ✓
  • P4, S8, Cl2: simple molecular, only weak id-id forces are overcome; S8 highest as it has the most electrons. ✓
  • Ar: single atoms with the weakest id-id forces; lowest. ✓
03
Explain
[3 marks]

Explain why the first ionisation energy (a) generally increases across Period 3, (b) falls from Mg to Al.

  • (a) Nuclear charge increases while shielding stays similar, so the outer electron is held more strongly. ✓
  • (b) Al's outer electron is in 3p, which is higher in energy than 3s and partly shielded by the 3s electrons, so less energy is needed to remove it. ✓
04
Explain
[2 marks]

Explain why Na+, Mg2+ and Al3+ decrease in radius.

  • All three have the configuration 1s2 2s2 2p6. ✓
  • The nuclear charge increases from 11 to 13, pulling the same number of electrons closer. ✓
05
Explain
[2 marks]

Explain why silicon is a semiconductor while aluminium is a good conductor.

  • Aluminium has three delocalised electrons per atom free to move. ✓
  • In silicon the electrons are held in covalent bonds; only a few gain enough energy to move, so it conducts slightly (more when heated). ✓
06
Recall
[2 marks]

State and explain the trend in electronegativity from sodium to chlorine.

  • Electronegativity increases. ✓
  • Increasing nuclear charge and decreasing atomic radius mean the bonding pair is attracted more strongly. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.