AS Chemistry 9701: Metallic Bonding and Melting Point Trends

(a) Describe the structure and bonding present in a typical metal such as sodium, in terms of the particles present and the forces holding them together. [2]

(b) The melting points of the period 3 metals are: sodium $98\,^\circ\text{C}$ , magnesium $650\,^\circ\text{C}$ , aluminium $660\,^\circ\text{C}$ . Explain why melting point increases from sodium to aluminium, referring to the charge on the metal ion and the number of delocalised electrons contributed by each atom. [3]

(c) The melting points of the group 1 metals are: lithium $181\,^\circ\text{C}$ , sodium $98\,^\circ\text{C}$ , potassium $64\,^\circ\text{C}$ . Explain why melting point decreases down group 1 from lithium to potassium, even though each atom contributes the same number of delocalised electrons in every case. [2]

Part (a): Structure and bonding in a metal

A metal such as sodium consists of a giant lattice of positive metal ions ( $\text{Na}^+$ ) arranged in a regular, repeating pattern. Each sodium atom releases its single outer-shell electron, and these released electrons are not attached to any particular ion. Instead they form a “sea” of delocalised electrons that is free to move throughout the whole structure.

The metallic bond is the strong electrostatic attraction between the lattice of positive ions and this delocalised sea of electrons, extending throughout the entire structure rather than being localised between any two specific atoms.

Part (b): Why melting point increases from Na to Al

Metal	Ion formed	Delocalised electrons per atom
Na	$\text{Na}^+$	$1$
Mg	$\text{Mg}^{2+}$	$2$
Al	$\text{Al}^{3+}$	$3$

Moving across period 3 from Na to Mg to Al, two things happen together:

Ionic charge increases ( $+1 \to +2 \to +3$ ), so each ion attracts the delocalised electron sea more strongly (a higher charge means a stronger electrostatic pull).
Number of delocalised electrons per atom increases ( $1 \to 2 \to 3$ ), increasing the density of the delocalised “sea” and adding more electrons that can be attracted to each ion.
The ionic radius also decreases slightly across the period (greater nuclear charge pulling the remaining electrons in more tightly), letting the ions and the delocalised electrons pack closer together, which further strengthens the attraction.

All three effects strengthen the metallic bonding from Na to Mg to Al, so progressively more thermal energy is required to overcome the electrostatic attraction and melt the lattice, consistent with the rising melting points ( $98\,^\circ\text{C} \to 650\,^\circ\text{C} \to 660\,^\circ\text{C}$ ).

Part (c): Why melting point decreases down group 1

Every group 1 atom has the same outer-shell electron configuration pattern (one electron in its outermost shell), so lithium, sodium and potassium each form a $+1$ ion and each contribute exactly $1$ delocalised electron per atom. This does not change down the group, so it cannot explain the trend.

What does change down the group is atomic (and ionic) radius, which increases from Li to Na to K as each successive element has one more electron shell. A larger ionic radius means the delocalised electrons are, on average, further away from the positive ion’s nucleus. Since electrostatic attraction weakens with increasing distance, this makes the metallic bond weaker down the group, so less thermal energy is needed to overcome it, consistent with the falling melting points ( $181\,^\circ\text{C} \to 98\,^\circ\text{C} \to 64\,^\circ\text{C}$ ).

Final answers

(a) A lattice of positive metal ions surrounded by a delocalised “sea” of electrons; the metallic bond is the electrostatic attraction between them.
(b) Melting point rises from Na to Al because both the ionic charge and the number of delocalised electrons per atom increase (with ionic radius also decreasing), strengthening the metallic bond.
(c) Melting point falls from Li to K because, although each atom always contributes $1$ delocalised electron, increasing atomic/ionic radius down the group weakens the electrostatic attraction to the delocalised electrons.