PeriodicityEdexcel A-Level Chemistry: Revision notes

Periodicity is a repeating pattern of properties across different periods of the periodic table. Elements in the same group have similar properties because they have the same outer electron configuration, so as the periods are crossed the same pattern is repeated: metals on the left, giant covalent elements near the middle, and simple molecules and noble gases on the right.

Periodicity is illustrated using data such as electronic configurations, atomic radii, melting and boiling temperatures and first ionisation energies.

The pattern depends on structure and bonding.

• Na, Mg, Al: giant metallic lattices. Attraction between positive ions and delocalised electrons. Melting temperature rises (98, 650, 660 °C) because the ion charge increases (1+, 2+, 3+), there are more delocalised electrons and the ions are smaller.
• Si: giant covalent. Many strong covalent bonds are broken on melting, so the melting temperature is the highest (1410 °C).
• P₄, S₈, Cl₂: simple molecular. Only weak London forces between molecules are broken, so melting temperatures are low (44, 115 and −101 °C). S₈ has the largest molecules, and so the strongest London forces.
• Ar: monatomic, the weakest London forces, and the lowest melting temperature (−189 °C).

Period 2 follows the same pattern. Lithium and beryllium are metals, with giant metallic lattices. Boron and carbon (diamond and graphite) have giant covalent structures, with the highest melting temperatures. Nitrogen (N₂), oxygen (O₂), fluorine (F₂) and neon (Ne) are simple molecular or monatomic with weak London forces, so they have very low melting and boiling temperatures. Bonds inside the molecule, such as the N≡N triple bond, are not broken on melting.

Across a period there is a general increase: the nuclear charge increases, the shielding stays about the same (electrons go into the same shell) and the atomic radius falls, so the outer electron is held more strongly.

Two small decreases occur in each period, which provide evidence for sub-shells and orbitals:

• Be to B (Mg to Al): the outer electron is in a p sub-shell, higher in energy than the s sub-shell (and, for Al, partly shielded by the 3s electrons).
• N to O (P to S): two electrons share one p orbital and the repulsion between them makes one easier to remove.

Period 3 data (atomic radius in nm, first ionisation energy in kJ mol⁻¹): Na 0.186 and 496; Mg 0.160 and 738; Al 0.143 and 578; Si 0.118 and 786; P 0.110 and 1012; S 0.103 and 1000; Cl 0.099 and 1251; Ar 1521.

The radii decrease steadily because the nuclear charge increases with similar shielding. The same pattern recurs in Period 2, which shows periodicity. Plots of melting temperature against atomic number rise to a maximum at Group 4 (carbon or silicon) and then fall, and the whole pattern repeats in the next period.