Chemists use symbols and formulae to represent elements, ions and compounds. Chemical equations model the changes that happen in chemical reactions.

Ionic compounds are made up of oppositely charged ions joined together by ionic bonds. They almost always contain at least one metal element and at least one non-metal element.

The formula of an ionic compound can be deduced from the formulae of its ions. Remember that for the metal elements in groups 1, 2, and 3 the charge on the ion can be deduced by how many outer shell electrons there were in the neutral atom. Similarly, the non-metal elements in groups 6 (IUPAC group 16) and 7 (IUPAC group 17), the ionic charge can be deduced by working out how many electrons must be gained to fill the outer shell.

Positive ions | Negative ions |
---|---|

lithium Li^{+} | chloride Cl^{-} |

sodium Na^{+} | bromide Br^{-} |

potassium K^{+} | iodide I^{-} |

silver Ag^{+} | oxide O^{2-} |

barium Ba^{2+} | |

calcium Ca^{2+} | |

copper(II) Cu^{2+} | |

iron(II) Fe^{2+} | |

lead(II) Pb^{2+} | |

magnesium Mg^{2+} | |

zinc Zn^{2+} | |

aluminium Al^{3+} | |

iron(III) Fe^{3+} |

Although ionic compounds contain electrically charged ions, they are neutral overall. The formula for an ionic compound must give the same number of positive and negative charges. Here are some examples.

**Example 1**

Sodium chloride contains Na^{+} and Cl^{-} ions:

- this is one positive charge and one negative charge
- the number of charges are already the same
- so the formula is
**NaCl**

**Example 2**

Magnesium oxide contains Mg2^{+} and O2^{-} ions:

- this is two positive charges and two negative charges
- the number of charges are already the same
- so the formula is
**MgO**

**Example 3**

Aluminium oxide contains Al^{3+} and O^{2-} ions:

- this is three positive charges and two negative charges
- to make the number of charges the same, we need two Al
^{3+}ions and three O^{2-}ions - so the formula is
**Al**_{2}O_{3}

Polyatomic ions are formed from groups of two or more atoms.

Positive ions | Negative ions |
---|---|

ammonium NH_{4}^{+} | hydroxide OH^{-} |

nitrate NO_{3}^{-} | |

carbonate CO_{3}^{2-} | |

sulfate SO_{4}^{2-} |

If the formula of an ionic compound needs more than one polyatomic ion, the formula of this ion is written inside brackets.

**Example**

Calcium hydroxide contains Ca^{2+} and OH^{-} ions:

- this is two positive charges and one negative charge
- to make the number of charges the same, we need one Ca
^{2+}ion and two OH^{-}ions - so the formula is
**Ca(OH)**_{2}

- Question
Deduce the formula for lead(II) sulfate.

Lead(II) nitrate contains Pb

^{2+}and SO_{4}^{2-}ions:- this is two positive charges and two negative charges
- the number of charges are already the same, and we only need one SO
_{4}^{2-}ion - so the formula is
**PbSO**_{4}

- Question
Deduce the formula for ammonium carbonate.

Ammonium carbonate contains NH

_{4}^{+}and CO_{3}^{2-}ions:- this is one positive charge and two negative charges
- to make the number of charges the same, we need two NH
_{4}^{+}ions and one CO_{3}^{2-}ion - so the formula is
**(NH**_{4})_{2}CO_{3}