Neutrons are neutral, but protons and electrons are electrically charged. Protons have a relative charge of +1, while electrons have a relative charge of -1.

The number of protons in an atom is called its atomic number. In the periodic table atoms are arranged in atomic number order.

Electrons are arranged in energy levels or shells, and different energy levels can hold different numbers of electrons. The electronic structure of an atom is a description of how the electrons are arranged, which can be shown in a diagram or by numbers. There is a link between the position of an element in the periodic table and its electronic structure.

Electrons are arranged in different shells around the nucleus. The innermost shell - or lowest energy level - is filled first. Each succeeding shell can only hold a certain number of electrons before it becomes full. The innermost shell can hold a maximum of two electrons, the second shell a maximum of eight, and so on. The table gives the maximum capacity of the first four shells - which is as much as you need to know at GCSE.

Energy level or shell Maximum number of electrons first 2 second 8 third 8 fourth 18

A lithium atom, for example, has three electrons. Two are in the first energy level, and one in the second.

A carbon atom has six electrons. Two are in the first energy level, and four in the second energy level.

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.

There is a quick way to work out what the charge on an ion should be:

• the number of charges on an ion formed by a metal is equal to the group number of the metal
• the number of charges on an ion formed by a non-metal is equal to the group number minus eight
• hydrogen forms H⁺ ions

When metals react with non-metals, electrons[electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] are transferred from the metal atoms to the non-metal atoms, forming ions[ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons. ]. The resulting compound is called an ionic compound[ionic compound: An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron): The ions swap electrons to achieve a full outer shell. ].

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.