Periodicity melting points and ionisation energies

Key Definition A periodic property is one that repeats at regular intervals when the elements are arranged in order of increasing atomic number; first ionisation energy and melting temperature are the clearest examples.

Plotting log(first ionisation energy) for elements 1 to 36 makes the periodic pattern visually clear, with peaks at the noble gases and troughs at the Group 1 metals.

Melting and boiling temperatures across Period 3 reflect the structure and bonding of each element.

The metals Na, Mg and Al show rising melting temperatures because the number of delocalised electrons per atom increases (1, then 2, then 3), while the cation charge increases and the ionic radius decreases, giving progressively stronger metallic bonding.

Silicon has the highest melting temperature in the period because its giant covalent (macromolecular) lattice requires many strong covalent bonds to be broken.

The simple molecular non-metals (P₄, S₈, Cl₂) and monatomic Ar have low melting temperatures because only weak London (induced-dipole) forces are overcome; of these S₈ melts highest and Ar lowest, following the number of electrons per particle.

First ionisation energy generally increases across a period because the nuclear charge rises while shielding stays roughly constant and the atomic radius decreases, so the outer electrons are held more strongly.

Two exceptions interrupt this rising trend across each period:

• The s-to-p dip (Be→B, Mg→Al): the electron removed from boron and aluminium comes from a p sub-shell, which lies at a higher energy than the filled s sub-shell of the preceding element, so it is more easily removed and the first ionisation energy falls slightly.
• The pairing dip (N→O, P→S): in oxygen and sulfur the outer p sub-shell now contains its first pair of electrons in one orbital, and the repulsion between these paired electrons makes one easier to remove, so the first ionisation energy falls slightly.

First ionisation energy decreases down a group because the outer electron is in a higher shell, further from the nucleus and more shielded by the inner shells, and this outweighs the increased nuclear charge.

The exam focus is structural reasoning. Always link a melting temperature to its bonding type, and link an ionisation energy trend to electron configuration, sub-shell energy, or shielding. Using the precise vocabulary — delocalised electrons, giant covalent, London forces, sub-shell, paired electrons, shielding — secures marks that vague answers miss.