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9701 · 9.1

Periodicity of physical properties of the elements in Period 3 — practice questions

Practice and worked examples for 9701 Periodicity of physical properties of the elements in Period 3. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

Explain why the first ionisation energy of Magnesium (12) is greater than that of Aluminium (13), despite Aluminium having a higher nuclear charge.

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  1. State electron configurations: Mg is 1s²2s²2p⁶3s² and Al is 1s²2s²2p⁶3s²3p¹.
  2. Identify orbitals: The electron removed from Mg is from the 3s orbital. The electron removed from Al is from the 3p orbital.
  3. Compare energy levels: The 3p orbital is at a higher energy level than the 3s orbital.
  4. Mention shielding: The 3p electron in Al is also shielded by the electrons in the 3s subshell.
  5. Conclusion: Less energy is required to remove the electron from the higher-energy, more-shielded 3p orbital of Aluminium compared to the 3s orbital of Magnesium, overriding the effect of the increased nuclear charge.

Worked example 2

Arrange sodium, silicon, and sulfur in order of increasing melting point. Justify your answer in terms of structure and bonding.

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  1. Order: The correct order of increasing melting point is Sodium < Sulfur < Silicon.
  2. Sodium (Na): Has a giant metallic lattice. Melting involves overcoming the electrostatic attraction between Na⁺ ions and a 'sea' of delocalised electrons. This requires a moderate amount of energy. M.P. = 98 °C.
  3. Sulfur (S): Exists as simple S₈ molecules. It has a low melting point because only weak van der Waals' forces between the S₈ molecules need to be overcome. The covalent bonds within the molecules remain intact. M.P. = 115 °C.
  4. Silicon (Si): Has a giant covalent (macromolecular) structure. Melting requires breaking many strong covalent bonds throughout the lattice, which demands a very large amount of energy. M.P. = 1414 °C.
  5. Conclusion: The order is Na < S < Si, due to the energy required to overcome metallic bonds, van der Waals' forces, and covalent bonds respectively.
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