Ionic bonds

Each sodium ion in a salt crystal has exactly six chloride neighbours, all 282 pm away.

Look at grains of table salt under a magnifying glass and many of them are tiny cubes. That shape is a clue to what is going on inside. Sodium chloride is not made of separate molecules. It is a regular three-dimensional grid of sodium ions and chloride ions, alternating in every direction, and the cube you can see is that grid repeated many millions of times.

Giving and taking electrons

A sodium atom has 11 electrons arranged 2, 8, 1. Its lone outer electron is held weakly, so sodium can lose it and be left with the full shell of neon. Having 11 protons but only 10 electrons, it becomes a positive ion, Na⁺. A chlorine atom has 17 electrons arranged 2, 8, 7, one short of a full shell. It takes an electron and becomes Cl⁻, with 18 electrons like argon. Sodium's electronegativity is 0.93 and chlorine's is 3.16. With a gap that wide the electron does not get shared; it changes owners.

Opposite charges attract, and that attraction is the ionic bond. It does not point in one direction. Each Na⁺ pulls on every Cl⁻ around it, and each Cl⁻ on every Na⁺. The most stable way to pack them is the rock-salt arrangement shown in the model: every ion has six neighbours of the opposite charge, placed above, below, left, right, in front and behind, at 90° to each other. The distance between neighbouring centres is 282 pm, half of the 564 pm edge of the repeating cube.

Writing the formula

Because there are no separate molecules, the formula of an ionic compound gives only the ratio of ions. NaCl means one sodium for every chlorine. The charges must balance to zero. Magnesium forms Mg²⁺, so it needs two chloride ions: MgCl₂. Aluminium forms Al³⁺ and oxygen forms O²⁻; the smallest whole numbers that balance +3 and −2 are two aluminium and three oxygen, Al₂O₃. Group 1 metals form 1+ ions, group 2 form 2+, group 17 nonmetals form 1− and group 16 form 2−.

What ionic bonding does to properties

Every ion is held by many neighbours, so pulling the crystal apart takes a lot of energy. Sodium chloride melts at 801 °C, far hotter than a kitchen oven. Solid salt does not conduct electricity, because the ions are locked in place. Melt it, or dissolve it in water, and the ions can move, so the liquid conducts. Ionic crystals are also brittle. Tap a salt crystal hard and one layer slides; suddenly like charges face each other, repel, and the crystal splits along a flat face.

The ions are not the same size as the atoms they came from. Losing its outer shell shrinks sodium to an ionic radius of about 102 pm, while the extra electron swells chloride to about 181 pm. In the space-filling view the large green chloride ions fill most of the space and the small purple sodium ions sit in the gaps between them.

When salt dissolves in water the lattice comes apart one ion at a time. Water molecules crowd around each ion: the slightly negative oxygen ends face the Na⁺ ions and the slightly positive hydrogen ends face the Cl⁻ ions. Surrounded like this, the ions drift away from the crystal and spread through the water. Because the dissolved ions can move, salty water conducts electricity, which is one reason why electrical equipment and seawater are a dangerous mix. You will see why water molecules have charged ends in the unit on polarity.