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What is ionic bonding?

Compounds that contain ions are called ionic compounds. These compounds are usually formed by a reaction between a metal and a non-metal.

Why do these substances react together and form bonds?

The metal and non-metal atoms have incomplete outer electron shells and so are unstable.

Electrons are transferred from each metal atom to each non-metal atom. The metal and the non-metal atoms form ions with completely full outer shells and become stable.

The positive and negative ions are strongly attracted to each other. This electrostatic attraction is called ionic bonding.

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How are ionic bonds formed?

Sodium chloride is an ionic compound formed by the reaction between the metal sodium and the non-metal chlorine.

Sodium has 1 electron in its outer shell.

Chlorine has 7 electrons in its outer shell.

2.8.7 [2.8.8]-

+

Cl Cl

-

2.8.1 [2.8]+

Na NaBy losing this electron, it has a filled outer shell and forms a positive ion.

By gaining an electron from sodium, it has a filled outer shell and forms a negative ion.

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How are ionic bonds formed?

The positive sodium ions and the negative chloride ions are strongly attracted to each other.

-

Cl

It is this electrostatic attraction that forms ionic bonds in sodium chloride and other ionic compounds.

Na

+

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Ions and ionic bonding – summary

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What is the ratio of ions in sodium oxide?

What is the ratio of ions needed to make sodium oxide?

Sodium (2.8.1) needs to lose 1 electron to form a sodium ion but oxygen (2.6) must gain 2 electrons to form an oxide ion .

Na

Na

O O2-

Na+

Na+

1 electron from each Na atom

Two sodium atoms are required for each oxygen atom and so the ratio of sodium ions to oxide ions is 2:1.

From this ratio, the formula of sodium oxide is Na2O.

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How to write the formula of an ionic compound

To work out the formula of an ionic compound:

1. Write down the symbol for each element – the metal is always written first.

2. Calculate the charge for each type of ion.

3. Balance the number of ions so that the positive and negative charges are balanced and equal zero. This gives the ratio of ions.

4. Use the ratio to write down the formula of the ionic compound.

The formula of a compound uses chemical symbols and numbers to show the ratio of atoms of each element present.

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Formula of aluminium bromide

What is the formula of aluminium bromide?

Al Br

+3 -1

AlBr3

3 bromide ions are needed for each aluminium ion

1 : 3

Symbol

Balance the number of ions

Ion charge

Ratio of ions

Formula

Al

Br

Al3+

Br

Br

Br-

Br-

Br-

1 electron for each bromine

atom

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What is the ionic formula?

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