Physical properties of Period 3 elementsAQA A-Level Chemistry: Revision notes

Atomic radius decreases from sodium to chlorine (0.186 nm to 0.099 nm).

• The nuclear charge increases by one proton for each element.
• The extra electrons go into the same shell (n = 3), so the shielding stays about the same.
• The outer electrons are therefore attracted more strongly to the nucleus and are pulled closer.

Argon is not usually included in this comparison, because it is monatomic and forms no covalent bonds, so its radius is measured in a different way.

The first ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. Across Period 3 there is a general increase (Na 496 to Ar 1521 kJ mol⁻¹) because:

• nuclear charge increases
• electrons are removed from the same shell with similar shielding
• atomic radius decreases, so the attraction is stronger.

There are two dips in the trend:

• Mg to Al (738 to 578): the outer electron of Al is in a 3p sub-shell, which is higher in energy and further from the nucleus than 3s, and is partly shielded by the 3s electrons. It is easier to remove.
• P to S (1012 to 1000): in S the 3p electrons begin to pair (3p⁴). The two electrons in the same orbital repel each other, so one is easier to remove than from P (3p³, all unpaired).

Sodium, magnesium and aluminium are metallic. Their lattices have positive ions attracted to delocalised electrons.

Melting point increases from Na to Al (371 K, 923 K, 933 K) because:

• the ionic charge increases, 1+, 2+, 3+
• the number of delocalised electrons per atom increases, 1, 2, 3
• the ions get smaller, so the charge density is higher.

The attraction between ions and electrons is stronger, so more energy is needed to overcome it.

Silicon has a giant covalent (macromolecular) structure, each atom bonded to four others by strong covalent bonds. Melting requires many of these bonds to be broken, so silicon has the highest melting point in the period (1687 K).

Phosphorus (P₄), sulfur (S₈), chlorine (Cl₂) and argon (Ar) are simple molecular (argon is monatomic). Melting only needs the weak London forces between the molecules to be overcome, not the covalent bonds within them. The melting points are low.

London forces get stronger with more electrons, so the order is S₈ > P₄ > Cl₂ > Ar: S₈ has 128 electrons, P₄ 60, Cl₂ 34 and Ar 18.

• Atomic radius: decreases Na to Cl as nuclear charge increases with similar shielding.
• First ionisation energy: general increase, with dips at Al (3p) and S (paired 3p electrons).
• Melting point: rises Na to Al (metallic), peaks at Si (giant covalent), then drops sharply to P₄, S₈, Cl₂ and Ar (London forces), with S₈ highest of these.