The p block

MDCAT Chemistry · Chemistry · s and p block elements. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The p block holds groups 13 to 18. Its best known family is the halogens (group 17): reactive non-metals with seven outer electrons that exist as diatomic molecules. Each atom gains one electron to form a halide ion, so halogens are oxidising agents. Down the group the atoms get bigger, the colour deepens, the boiling point rises and the oxidising power falls.

Across period 3 the compounds change with the bonding. Metal oxides are ionic and basic, non-metal oxides are covalent and acidic, and aluminium oxide lies between them as amphoteric. The chlorides change from neutral, ionic NaCl to covalent chlorides that water hydrolyses.

Metals occur in nature as ores, mostly oxides, sulphides and carbonates, and are extracted by reduction.

Rules to remember

Common mistake

Students write that HF is the strongest halogen acid because fluorine is the most electronegative. HF is the weakest, because its very strong H–F bond is hard to break in water.

Worked example

Chlorine is passed into hot concentrated sodium hydroxide: 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O. What happens to the oxidation number of chlorine?

  1. In Cl2, an element, chlorine is 0.
  2. In NaCl it is −1, so five chlorine atoms are reduced, each gaining 1 electron.
  3. In NaClO3: (+1) + x + 3(−2) = 0, so x = +5. One chlorine atom is oxidised, losing 5 electrons.
  4. Electrons gained (5 × 1) = electrons lost (1 × 5), so the equation is balanced.

Answer: Chlorine goes from 0 to −1 and to +5: it is reduced and oxidised at once (disproportionation).

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