Formulae (Cambridge (CIE) A Level Chemistry): Revision Note

Formulae of Ionic Compounds

• Ionic compounds are formed from a metal and a non-metal bonded together

• Ionic compounds are electrically neutral; the positive charges equal the negative charges

Charges on positive ions

• All metals form positive ions

  • There are some non-metal positive ions such as ammonium, NH₄⁺, and hydrogen, H⁺

• The metals in Group 1, Group 2 and Group 13 have a charge of 1+ and 2+ and 3+ respectively

• The charge on the ions of the transition elements can vary which is why Roman numerals are often used to indicate their charge

• Roman numerals are used in some compounds formed from transition elements to show the charge (or oxidation state) of metal ions

  • E.g. in copper (II) oxide, the copper ion has a charge of 2+ whereas in copper (III) nitrate, the copper has a charge of 3+

Non-metal ions

• The non-metals in Group 15 to 17 have a negative charge and have the suffix ‘ide’

  • E.g. nitride, chloride, bromide, iodide

• Elements in Group 17 gain 1 electron so have a 1- charge, eg. Br^-

• Elements in Group 16 gain 2 electrons so have a 2- charge, eg. O^2-

• Elements in Group 15 gain 3 electrons so have a 3- charge, eg. N^3-

• There are also more complex negative ions, which are negative ions made up of more than one type of atom

 Periodic table outlining common ion charges

Formulae of ions

• Silver(I); Ag⁺

• Ammonium: NH₄⁺

• Zinc(II): Zn²⁺

• Hydroxide: OH^–

• Nitrate: NO₃^–

• Sulfate: SO₄^2–

• Carbonate: CO₃^2–

• Hydrogen carbonate: HCO₃^–

• Phosphate: PO₄^3–