Constructing ionic formulae

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.

Reactions between metals and non-metals include:

  • sodium + chlorine → sodium chloride
  • magnesium + oxygen → magnesium oxide
  • calcium + chlorine → calcium chloride
  • potassium + oxygen → potassium oxide
  • aluminium + oxygen → aluminium oxide

The details of how ions are formed are not required for this topic. In Unit 1 you are expected to be able to use the table of common ions to identify the ions present in a compound and then construct the formula of the compound from the ions.

Sodium chloride

  • Identify the ions – sodium chloride contains sodium ions (Na+) and chloride ions (Cl).
  • Cancel out the charges – the overall charge of an ionic compound must be zero. This means the same number of positive charges as negative charges is needed. Since the sodium has one positive charge and chloride has one negative charge, only one of each is needed to cancel the charges.
  • Construct the formula – one of each ion is needed. This leads to a formula of NaCl.

Sometimes an ionic formula contains compound ions – a group of atoms bonded together with an overall charge.

Magnesium hydroxide

  • Identify the ions – magnesium hydroxide contains magnesium ions (Mg2+) and hydroxide ions (OH).
  • Cancel out the charges – the overall charge of an ionic compound must be zero. In this case, magnesium has two positive charges which means that two hydroxide groups are needed to cancel out the charges completely.
  • Construct the formula – one magnesium ion and two hydroxide ions are needed. As hydroxide ions are made of a group of atoms, we must put them in brackets in the formula. This leads to a formula of Mg(OH)2.

Other compound ions that are used are:

  • carbonate (CO32–)
  • sulfate (SO42–)
  • nitrate (NO3)
  • ammonium (NH4+)

Iron(III) oxide

Firstly, identify the ions – iron(III) oxide contains iron(III) ions (Fe3+) and oxide ions (O2–).

Next, cancel out the charges – the overall charge of an ionic compound must be zero. This means we have to have the same number of positive charges as negative charges. Since the iron has three positive charges and the oxide has two negative charges, the only way to cancel the charges is to have six of each. This means there need to be two iron ions and three oxide ions.

Example

Diagram to show how two iron(III) ions and three oxide ions cancel out the charges in iron(III) oxide.

Finally, construct the formula – two iron ions and three oxide ions are needed. This leads to a formula of Fe2O3.

Charges of the ions in the formula of the compound are not written. The number of ions of each element is written as a subscript number after the atomic symbol.

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