Giant Ionic Lattice: Structure and Properties of NaCl

A giant ionic lattice is a repeating 3D arrangement of positive and negative ions held together by strong electrostatic attraction. In sodium chloride (NaCl) every Na⁺ ion is surrounded by six Cl⁻ ions, and every Cl⁻ by six Na⁺ ions, so the pattern runs through the whole crystal. This one structure explains why salt is hard, brittle, has a high melting point and conducts only when molten or dissolved.

What does the sodium chloride structure look like?

Sodium chloride is built on a face-centred cubic pattern of ions. Each ion sits at a point on a cubic grid, with positive and negative ions alternating in every direction. The number of nearest neighbours of the opposite charge is the coordination number, and for NaCl it is 6:6.

Not every ionic compound packs the same way. Caesium chloride has a coordination number of 8:8, because the Cs⁺ ion is larger and fits eight chloride ions around it. The ratio of the ion sizes decides which pattern is most stable.

There are no separate NaCl molecules. The formula NaCl is only the simplest ratio of ions, 1 Na⁺ to 1 Cl⁻, which is why chemists call it a formula unit.

Why do ionic compounds have high melting points?

To melt the crystal you must give the ions enough energy to break free of their fixed positions. Each ion is held by strong attraction to six oppositely charged neighbours, so a lot of energy is needed. NaCl melts at 801 °C and boils at 1413 °C.

The size of the charges matters. In magnesium oxide, Mg²⁺ and O²⁻ each carry a 2+ or 2− charge, so the attraction is much stronger than in NaCl (+1 and −1). MgO melts at about 2852 °C. A higher charge and a smaller ion both give a stronger lattice and a higher melting point.

Do ionic compounds conduct electricity?

A substance conducts when charged particles are free to move.

State of NaCl	Ions free to move?	Conducts?
Solid	No, locked in the lattice	No
Molten (above 801 °C)	Yes	Yes
Dissolved in water	Yes	Yes

This is also the basis of electrolysis, where molten or dissolved ions move to the electrodes.

Why are ionic crystals brittle?

Hit a salt crystal and one layer of ions slides by a single position. Now like charges sit next to each other, repel, and the crystal cracks along a flat plane. Metals behave differently because their layers can slide while the sea of delocalised electrons keeps them bonded, so they bend instead.

Why does salt dissolve in water?

Water molecules are polar. The slightly negative oxygen end is attracted to Na⁺ and the slightly positive hydrogen end to Cl⁻. Water pulls ions out of the lattice and surrounds each one, which is called hydration. Compounds with very strong lattices, such as MgO, do not dissolve well because water cannot supply enough energy to break them apart.

How does an ionic lattice compare with diamond and graphite?

All of these are giant structures, but the bonds differ.

• Ionic lattice (NaCl): attraction between ions; high melting point; conducts when molten.
• Giant covalent (diamond): each carbon bonds covalently to four others; extremely hard; does not conduct.
• Giant covalent (graphite): each carbon bonds to three others in sheets; layers slide; conducts because of delocalised electrons.
• Giant metallic (copper): metal ions in a sea of electrons; bends; conducts when solid.

Common mistakes

• Calling NaCl a molecule. It is a lattice of ions with no discrete molecules.
• Saying ionic solids conduct because they contain ions. The ions must be free to move.
• Saying ionic bonds are weak when the solid shatters. The bonds are strong; the crystal is brittle because of repulsion between like charges after a layer shifts.
• Drawing only one ion's neighbours. The lattice repeats in all three dimensions.

Frequently asked questions

What is a giant ionic lattice?

A giant ionic lattice is a regular, repeating 3D arrangement of positive and negative ions held together by strong electrostatic forces. Each ion is surrounded by ions of opposite charge. Sodium chloride is the standard example: every Na⁺ has six Cl⁻ neighbours and every Cl⁻ has six Na⁺ neighbours.

Why do ionic compounds have high melting points?

Melting an ionic compound means breaking the strong electrostatic attraction between many ions at once. This takes a large amount of energy, so melting points are high: 801 °C for sodium chloride and about 2852 °C for magnesium oxide, where the ions carry larger charges.

Do ionic compounds conduct electricity?

Only when the ions can move. Solid ionic compounds do not conduct because the ions are fixed in the lattice. Molten ionic compounds and their solutions in water conduct, because the ions are free to carry charge to the electrodes.

Why are ionic compounds brittle?

When a force shifts one layer of ions along by one position, ions of the same charge end up next to each other. They repel, and the crystal splits cleanly along that plane. This is why salt crystals shatter rather than bend.

Try it yourself

Open the Crystal Lattice Viewer and choose sodium chloride. Count the six neighbours around one ion, then hit the crystal with the hammer to watch the layers split. Raise the temperature to melt it, then use the battery to see that it conducts only once the ions can move. For how the ions form in the first place, read how do ions form and ionic vs covalent bonds.