Periodicity

Periodicity

Periodicity is the repeating pattern of properties across the periods of the periodic table, caused by the repeating pattern of electron configurations. The table is arranged into s, p, d and f blocks according to which sub-shell the outer electrons occupy.

Atomic radius

Decreases across a period: more protons (greater nuclear charge) pull the electrons — in the same shell — closer. Increases down a group: more shells and more shielding.

First ionisation energy

General increase across a period (greater nuclear charge, same shielding), with two dips:

• Between Group 2 and 3 (e.g. Mg → Al): the electron removed from Al is in a higher-energy p sub-shell, so it's easier to remove.
• Between Group 5 and 6 (e.g. N → O): in O, two electrons pair up in a p orbital and repel, making one easier to remove.

Melting points across Period 3

The trend reflects structure and bonding:

• Na → Al: giant metallic — melting point rises (more delocalised electrons, stronger metallic bonding).
• Si: giant covalent (macromolecular) — very high melting point (many strong covalent bonds).
• P, S, Cl: simple molecular — low melting points (weak van der Waals forces); sulfur (S₈) is higher than phosphorus (P₄) because it's a larger molecule.
• Ar: monatomic — the lowest melting point.

Reactions of Period 3 elements (outline)

• With oxygen: metals form basic/amphoteric oxides; non-metals form acidic oxides.
• With water: sodium reacts vigorously (→ alkaline solution); magnesium reacts slowly.

Worked example

Explain why the melting point of silicon is much higher than that of phosphorus.

• Silicon has a giant covalent structure, so melting requires breaking many strong covalent bonds. Phosphorus is simple molecular (P₄) with only weak van der Waals forces between molecules, which need far less energy to overcome. ✓

Common mistakes

• Forgetting the two dips in first ionisation energy (Group 2→3 and 5→6) and their reasons.
• Attributing the Period 3 melting-point trend to anything other than structure and bonding.
• Saying atomic radius increases across a period — it decreases.

Exam tips

• Explain atomic radius and ionisation trends using nuclear charge, shielding and sub-shell.
• For melting points, always name the structure and bonding (metallic / giant covalent / simple molecular).
• Learn the reasons for the ionisation-energy dips.

Key facts to remember

• Periodicity comes from repeating electron configurations; blocks s/p/d/f.
• Across a period: atomic radius decreases, first ionisation energy generally increases (dips at Group 2→3 and 5→6).
• Period 3 melting points: metallic (Na–Al) rise, Si giant covalent = very high, simple molecular (P, S, Cl) low, Ar lowest.