Period 3 Structure and Bonding: Na to Ar

Learning Objectives (OCR Spec 3.1.1)

By the end of this lesson you should be able to:

• Describe the structure and bonding of the Period 3 elements Na, Mg, Al, Si, P, S, Cl, Ar
• Explain the trend in melting point across Period 3 in terms of structure and bonding
• Distinguish between giant metallic, giant covalent and simple molecular structures
• Relate physical properties to the forces that must be overcome on melting
• Explain why the melting point drops sharply between Si and P

Overview

graph LR
  A[Na] --> B[Mg]
  B --> C[Al]
  C --> D[Si]
  D --> E[P]
  E --> F[S]
  F --> G[Cl]
  G --> H[Ar]
  A -.->|Giant metallic| C
  D -.->|Giant covalent| D
  E -.->|Simple molecular P4| H

Period 3 contains three structure types: giant metallic (Na, Mg, Al), giant covalent (Si) and simple molecular (P4, S8, Cl2, Ar). The change from giant to simple structure causes a sharp drop in melting point between Si and P.

Melting Points Across Period 3

Element	Structure	Bonding	m.p. / degC
Na	Giant metallic	Metallic (1 delocalised e-)	98
Mg	Giant metallic	Metallic (2 delocalised e-)	650
Al	Giant metallic	Metallic (3 delocalised e-)	660
Si	Giant covalent	Covalent network	1410
P	Simple molecular (P4)	London forces	44
S	Simple molecular (S8)	London forces	113
Cl	Simple molecular (Cl2)	London forces	-101
Ar	Monatomic	London forces	-189

Na, Mg, Al: Giant Metallic

All three are giant metallic lattices with delocalised electrons. Moving Na → Mg → Al:

• Cation charge increases (+1 → +2 → +3)
• Number of delocalised electrons per atom increases (1 → 2 → 3)
• Ionic radius decreases

All three effects strengthen the metallic bond, so the melting point rises sharply from Na (98 degC) to Mg (650 degC) to Al (660 degC).

Si: Giant Covalent

Silicon has the same diamond-like structure as carbon in diamond: each Si atom is covalently bonded to four others in a giant 3D network. To melt Si, you must break strong covalent bonds throughout the lattice, which requires enormous energy. Hence Si has the highest melting point in Period 3 (~1410 degC).

P, S, Cl, Ar: Simple Molecular

From phosphorus onwards, the structure changes abruptly to simple molecular:

Element	Molecular formula	Shape
P	P4	Tetrahedral
S	S8	Crown (puckered ring)
Cl	Cl2	Diatomic
Ar	Ar (monatomic)	Spherical atoms

Within each molecule, atoms are held by strong covalent bonds, but between molecules there are only weak London dispersion forces (sometimes called instantaneous dipole-induced dipole forces). Melting requires only overcoming these weak intermolecular forces, so the melting points are all low.

The order of m.p. within the simple molecular group:

S8 (113 degC) > P4 (44 degC) > Cl2 (-101 degC) > Ar (-189 degC)

is determined by the number of electrons (and hence the strength of London forces) in each molecule. S8 has the most electrons (128 per molecule) and the strongest London forces; Ar has only 18 electrons per atom and the weakest London forces.