CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.2.3 Formulae

Formulae of Ionic Compounds

  • Ionic compounds are formed from a metal and a nonmetal 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
    • Eg. 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
    • Eg. 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. O2-
  • Elements in group 15 gain 3 electrons so have a 3- charge, eg. N3-
  • There are also more complex negative ions, which are negative ions made up of more than one type of atom


Atoms, Molecules & Stoichiometry Ionic Charges, downloadable AS & A Level Chemistry revision notes

The charges of simple ions depend on their position in the Periodic Table

Formulae of ionic compounds table

Atoms, Molecules & Stoichiometry Formulae of Ionic Compounds Table, downloadable AS & A Level Chemistry revision notes

Worked Example: Formulae

Atoms, Molecules & Stoichiometry Worked Example - Formulae, downloadable AS & A Level Chemistry revision notes


Answer 1: Magnesium chloride

  • Magnesium is in Group 2 so has a charge of 2+
  • Chlorine is in group 17 so has a charge of 1-
  • Magnesium needs two chlorine atoms for each magnesium atom to be balanced so the formula is MgCl2

Answer 2: Iron (III) oxide

  • The Roman numeral states that iron has a charge of 3+
  • Oxygen is in group 16 so has a charge of 2-
  • The charges need to be equal so 2 iron to 3 oxygen atoms will balance electrically, so the formula is Fe2O3

Answer 3: Aluminum nitrate

  • Aluminium is in group 13 so has a charge of 3+
  • Nitrate is a compound ion and has a charge of 1-
  • The complex ion needs to be placed in a bracket if more than 1 is needed
  • The formula of aluminium nitrate is Al(NO3)3

Exam Tip

Remember: compound ions are ions that contain more than one type of element, such as OH

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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