Giant ionic lattices: sodium chloride and magnesium oxide

By the end of this lesson

By the end you can describe the lattice structure of a giant ionic solid such as sodium chloride or magnesium oxide.

Not molecules — a lattice

An ionic compound does not exist as separate molecules. Its ions pack into a regular, repeating pattern called a lattice.

In sodium chloride ( $\text{NaCl}$ ):

positive $\text{Na}^+$ ions and negative $\text{Cl}^-$ ions alternate in every direction.
each $\text{Na}^+$ is surrounded by 6 $\text{Cl}^-$ ions — and each $\text{Cl}^-$ by 6 $\text{Na}^+$ ions.
the pattern repeats millions of times: it is one giant structure, so we call it a giant ionic lattice.
the formula $\text{NaCl}$ is a ratio (1 : 1), not a molecule.

Definition

Giant ionic lattice

A giant ionic lattice is a regular, repeating arrangement of positive and negative ions, held together by strong electrostatic attraction between oppositely charged ions throughout the structure.

Why ionic solids are brittle

The ions sit in tidy rows of alternating charge. Hit the crystal hard and one row slides slightly over the next.

Now like charges line up: $+$ beside $+$ , $-$ beside $-$ . They repel, and the crystal cracks along that plane.

Where the analogy helps: two magnets stuck in an alternating line — slide one and the line breaks apart.

Magnesium oxide: same shape, stronger grip

$\text{MgO}$ has the same lattice shape as $\text{NaCl}$ , but its ions carry double the charge: $\text{Mg}^{2+}$ and $\text{O}^{2-}$ .

The electrostatic attraction is therefore much stronger, so more energy is needed to break the lattice:

	ion charges	melting point
$\text{NaCl}$	$1+$ and $1-$	$801\ ^\circ\text{C}$
$\text{MgO}$	$2+$ and $2-$	about $2800\ ^\circ\text{C}$

Both are hard, high-melting and brittle — the classic signs of ionic structure.

Quick check

Fill in the blanksQuestion 1 · Recall

Multiple choiceQuestion 2 · Apply

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