Ionic compounds

The ionic bond

When a metal reacts with a non-metal, electrons move from the metal atoms to the non-metal atoms (2D). The positive metal ions and negative non-metal ions are strongly attracted to each other because opposite charges attract. This attraction is called an ionic bond, and a substance made of ions is an ionic compound.

The ions don't pair up one to one. Each ion attracts every oppositely charged ion around it, and repels ions with the same charge. The arrangement that balances all these pushes and pulls is a regular, repeating 3D pattern called a crystal lattice. In sodium chloride, every Na⁺ ion is surrounded by six Cl⁻ ions, and every Cl⁻ by six Na⁺. That cubic lattice is why salt crystals are cubes.

Properties of ionic compounds

• High melting points. Every ion is held by strong attractions in all directions, so a lot of energy is needed to break the lattice apart. Sodium chloride melts at 801 °C; magnesium oxide, with doubly charged ions, melts at about 2850 °C and is used to line furnaces.
• Hard but brittle. The lattice resists being scratched, but a sharp blow can shift one layer of ions along by one position. Suddenly positive ions sit next to positive ions and negative next to negative. They repel, and the crystal splits along a flat surface. You can hear this in a salt grinder.
• Conduct electricity only when molten or dissolved. In a solid the ions are locked in place. Melt the compound or dissolve it in water and the ions are free to move, so they can carry a current (2G).
• Many dissolve in water, which pulls the ions out of the lattice, though some, such as calcium carbonate (limestone), barely dissolve at all.

Writing ionic formulas

An ionic compound has no overall charge, so the total positive charge must equal the total negative charge. The formula shows the simplest ratio of ions that does this.

1. Write the two ions with their charges.
2. Work out how many of each you need so the charges add to zero.
3. Write the symbols, metal first, with the number of each ion as a subscript. Leave out the 1s and the charges.

Worked examples.

• Sodium chloride: Na⁺ and Cl⁻. One of each balances (+1 −1 = 0), so NaCl.
• Magnesium chloride: Mg²⁺ and Cl⁻. Two chloride ions balance one magnesium ion (+2 −2 = 0), so MgCl₂.
• Aluminium oxide: Al³⁺ and O²⁻. The smallest number both 3 and 2 go into is 6: two Al³⁺ (+6) and three O²⁻ (−6), so Al₂O₃.

A shortcut called the swap-and-drop (or criss-cross) method gives the same result: the number in each charge becomes the subscript of the other ion. Al³⁺ O²⁻ → Al₂O₃. Always check the result can't be simplified: Mg²⁺ and O²⁻ give Mg₂O₂, which simplifies to MgO.

Polyatomic ions

Some ions are groups of atoms that stay together and carry a charge as one unit. These polyatomic ions are worth learning:

Treat them like any other ion, but if you need more than one, put the whole ion in brackets: calcium hydroxide is Ca(OH)₂, ammonium carbonate is (NH₄)₂CO₃. Without brackets, CaOH₂ would mean one oxygen and two hydrogens, a different thing altogether.

Naming ionic compounds

Name the positive ion first, then the negative ion: sodium chloride, magnesium oxide, potassium nitrate, ammonium sulfate. Simple negative ions end in -ide; most ions ending in -ate contain oxygen. For metals with more than one ion, include the Roman numeral: iron(III) oxide is Fe₂O₃ (rust), copper(II) sulfate is CuSO₄.

Ionic compounds you meet every day include sodium chloride (table salt), sodium hydrogen carbonate (bicarb soda), calcium carbonate (chalk, marble, eggshells and coral), magnesium sulfate (Epsom salts) and sodium fluoride (in toothpaste). Kati Thanda–Lake Eyre in South Australia, Australia's largest salt lake, is crusted with ionic compounds left behind as the water evaporates; the salt crust can be up to about 50 cm thick.