Physical Properties of Period 3 Elements

Specification Reference Inorganic chemistry, Periodicity 3.2.1.2

Quick Notes

Atomic radius decreases across period 3 due to increasing nuclear charge with the same (or similar) level of electron shielding.
First ionisation energy increases as a trend across period 3 due to stronger attraction between the nucleus and outer electrons.
Melting points vary across period 3 due to different bonding and structures:
- Metals (Na, Mg, Al) → High melting points due to strong metallic bonds.
- Silicon (Si, giant covalent) → Very high melting point due to a giant covalent structure.
- Non-metals (P, S, Cl, Ar) → Low melting points due to simple molecular structures and weak intermolecular forces between molecules.

Full Notes

The period 3 elements are sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl) and argon (Ar).

AQA A-Level Chemistry list banner of Period 3 elements Na to Ar

They are in period 3 because atoms of each element have three occupied energy levels (shells).

Trend in Atomic Radius Across Period 3

Atomic radius decreases from Na to Ar.

AQA A-Level Chemistry diagram showing atomic radius decreasing from Na to Ar across Period 3 with measured pm values

Reason:

Nuclear charge increases (more protons in the nucleus).
Electron shielding remains the same or similar (all outer electrons are in the same energy level).
Stronger attraction pulls outer electrons closer.

Trend in First Ionisation Energy Across Period 3

First ionisation energy = Energy required to remove 1 mole of electrons from 1 mole of gaseous atoms.

First ionisation energy generally increases across the period.

AQA A-Level Chemistry line graph showing a general increase in first ionisation energy from Na to Ar with small dips

Reason for the trend:

Nuclear charge increases (more protons).
Atomic radius decreases, meaning outer electrons are closer to the nucleus.
Electrons are more strongly attracted, requiring more energy to remove.

AQA A-Level Chemistry comparison of Na and Cl electron shells illustrating stronger nuclear attraction and smaller radius toward the right

Exceptions to the trend (see first ionisation energy and atomic structure):

Aluminium (Al) has a lower ionisation energy than Mg.
Reason: The outer electron in Al is in the 3p subshell, which is higher in energy than the outer electron in Mg (3s) and easier to remove.
Sulfur (S) has a lower ionisation energy than phosphorus (P).
Reason: In sulfur, the 3p orbital starts to pair electrons, causing electron–electron repulsion, making it easier to remove an electron.

Trend in Melting Points Across Period 3

Melting points vary due to different bonding and structures.

Metals (Na, Mg, Al) → High melting points due to strong metallic bonding.
Silicon (Si) → Highest melting point due to its giant covalent structure.
Non-metals (P, S, Cl, Ar) → Lower melting points due to weak intermolecular forces.

AQA A-Level Chemistry chart of melting point changes across Period 3 with regions labelled metallic, giant covalent, simple molecular, monoatomic

Explanation of melting point trends:

Metals (Na, Mg, Al) have high melting points
Metallic bonding increases in strength from Na → Al due to more delocalised electrons and greater positive charge of metal ions.
Silicon (Si) has the highest melting point
Giant covalent structure with strong covalent bonds requires a lot of energy to break.
Non-metals (P₄, S₈, Cl₂, Ar) have low melting points
Weak van der Waals forces hold molecules together. S₈ has a higher melting point than P₄ and Cl₂ because it is a larger molecule, meaning stronger van der Waals forces.

Matt’s exam tip

Don’t forget that sulfur has a slightly higher melting point than phosphorus. This is because sulfur exists as molecules of S₈, whereas phosphorus is most commonly found as P₄ molecules. S₈ molecules are larger than P₄ molecules meaning stronger van der Waals forces and a higher melting point.

Summary

Across Period 3, atomic radius decreases as nuclear charge increases with similar shielding.
First ionisation energy generally increases; dips occur at Al (3p electron) and S (paired 3p electrons).
Melting points reflect structure and bonding: metals high (strong metallic), Si very high (giant covalent), non-metals low (weak intermolecular forces); S₈ > P₄ > Cl₂ ≫ Ar.