Ions are electrically charged particles formed when atoms lose or gain electrons. They have the same electronic structures as noble gases.

Metal atoms form positive ions, while non-metal atoms form negative ions. The strong electrostatic forces of attraction between oppositely charged ions are called ionic bonds.

Ionic compounds have high melting and boiling points.

How ions form

Ions are electrically charged particles formed when atoms lose or gain electrons. This loss or gain leaves a complete highest energy level, so the electronic structure of an ion is the same as that of a noble gas - such as a helium, neon or argon.

Metal atoms and non-metal atoms go in opposite directions when they ionise:

• Metal atoms lose the electron, or electrons, in their highest energy level and

become positively charged ions

• Non-metal atoms gain an electron, or electrons, from another atom to

become negatively charged ions

When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms, forming ions. The resulting compound is called an ionic compound.

Consider reactions between metals and non-metals, for example:

Sodium + chlorine → sodium chloride

Magnesium + oxygen → magnesium oxide

• Calcium + chlorine → calcium chloride

In each of these reactions, the metal atoms give electrons to the non-metal atoms. The metal atoms become positive ions and the non-metal atoms become negative ions.

There is a strong electrostatic force of attraction between these oppositely charged ions, called an ionic bond. The animation shows ionic bonds being formed in sodium chloride, magnesium oxide and calcium chloride.

There are many ionic bonds in an ionic compound such as sodium chloride, arranged in giant lattice structures. Ionic compounds have high melting and boiling points.

Ionic bonds are the electrostatic forces of attraction between oppositely-charged ions. The oppositely-charged ions are arranged in a regular way to form a giant ionic lattice. It is a 'lattice' because the arrangement is a regular one and 'giant' because the arrangement is repeated many times with large numbers of ions.

Ionic compounds often form crystals as a result. The diagram shows part of a sodium chloride ionic lattice.

The melting point of sodium chloride is lower than that of magnesium oxide because it has weaker ionic bonds, which need less heat energy to overcome. This is for two reasons:

• The Na⁺ and Cl^– ions in sodium chloride have fewer charges than the Mg²⁺and O^2– ions in magnesium oxide
• Na⁺ ions are larger than Mg²⁺ and cannot get as close to the negatively charged ions

For an ionic substance to conduct electricity, its ions must be free to move so that they can carry charge from place to place. Ions are free to move when an ionic compound is a molten liquid or in solution but not when it is solid.

The formation of ionic compounds is often shown with dot and cross models.

For example, a sodium atom loses one electron from its outer shell to become a positively charged sodium ion. It can be represented in a diagram like this: