Ions and their formation

In a nutshell

Ions are formed by the loss or gain of electrons. If an atom loses an electron a positively charged ion is formed, whereas if an atom gains an electron a negatively charged ion is formed.

Ion

Definition

An ion forms when an atom/group of atoms loses or gains electrons to form a charge.

An ion will lose or gain electrons to obtain a stable 'full' outer shell. The charge is a result of the unequal number of positively charged protons and negatively charged electrons.

Cations and anions

type of ion

electron transfer

CHARGE

protons compared to electrons

groups most likely to form

Cation

Loss of electrons

Positive

More protons than electrons

Group $1$

Group $2$

Anion

Gain of electrons

Negative

Less protons than electrons

Group $6$

Group $7$

Ions formed by different groups

Elements in a group all have the same number of outer shell electrons and will form the same type of ions.

Ions formed by group $1$ and $2$ metals

Lose electrons to form cations
Group $1$ elements lose one electron to form $1+$ charged ions
Group $2$ elements lose two electrons to form $2+$ charged ions

Ions formed by group $6$ and $7$ non-metals

Gain electrons to form anions
Group $7$ elements gain one electron to form $1-$ charged ions
Group $6$ elements gain two electrons to form $2-$ charged ions

Example

Explain why group $6$ elements form ions with a $2-$ charge.

Group $6$ elements have six electrons in their outer shell and want to gain two more electrons to fill their outer shell. The gain of two electrons leaves group $6$ elements with a $2-$ charge.

Writing ion formulae

A '+' or '-' is placed next to the formulae of the atom or group of atoms. This can be used to work out the formula for an ionic compound.

Example

Write the formula for the neutral ionic compound, sodium chloride.

Sodium is in group $1$ and will form a single positively charged ion - $Na^+$ . Chlorine is in group $7$ and will form a single negatively charged ion - $Cl^-$ . The single positive and negative charge will cancel out to form the neutral ionic compound, so only one of each type of atom is needed. The ionic formula is $\underline{NaCl}$ .