CIE IGCSE Chemistry

Revision Notes

4.1.1 Symbols & Formulae

Symbols & Formulae of Elements & Compounds

Element symbols

  • Each element is represented by its own unique symbol as seen on the Periodic Table e.g. H is hydrogen
  • Where a symbol contains two letters, the first one is always in capital letters and the other is small, eg. sodium is Na, not NA
  • Atoms combine together in fixed ratios that will give them full outer shells of electrons
  • The chemical formula tells you the ratio of atoms
  • Eg. H2O is a compound containing 2 hydrogen atoms which combine with 1 oxygen atom
  • The chemical formula can be deduced from the relative number of atoms present
  • Eg. if a molecule contains 3 atoms of hydrogen and 1 atom of nitrogen then the formula would be NH3
  • Diagrams or models can also be used to represent the chemical formula


Ammonia-Displayed-Formula, IGCSE & GCSE Chemistry revision notesThe ammonia molecule consists of a central nitrogen atom bonded to 3 hydrogen atom


Chemical formulae

  • The structural formula tells you the way in which the atoms in a particular molecule are bonded. This can be done by either a diagram (displayed formula) or written (simplified structural formula)
  • The empirical formula tells you the simplest whole-number ratio of atoms in a compound
  • The molecular formula tells you the actual number of atoms of each element in one molecule of the compound or element e.g. H2 has 2 hydrogen atoms, HCl has 1 hydrogen atom and 1 chlorine atom


Example: Butane

  • Structural formula (displayed)

Butane-Displayed-Formula, IGCSE & GCSE Chemistry revision notes


  • Structural formula (simplified)
  • Molecular formula
  • Empirical formula


Deducing formulae by combining power

  • The concept of valency is used to deduce the formulae of compounds
  • Valency or combing power tells you how many bonds an atom can make with another atom
  • Eg. carbon is in Group IV so a single carbon atom can make 4 single bonds or 2 double bonds
  • The following valencies apply to elements in each group:

Stoichiometry table, IGCSE & GCSE Chemistry revision notes


  • We can use the combining power of each atom to work out a formula
  • Example: what is the formula of aluminium sulfide?
    • Write out the symbols of each element and write their combining powers underneath:
    • Al         S
      3          2
  • The formula is then calculated by cross multiplying each atom with the number opposite, hence the formula for aluminium sulfide is Al2S3
Extended Only

Deducing Formulae of Ionic Compounds

  • The formulae of these compounds can be calculated if you know the charge on the ions.
  • Below are some common ions and their charges:

Ion, Formula And Charge table, IGCSE & GCSE Chemistry revision notes


  • For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges = 0
  • Example: what is the formula of aluminium sulfate?
    • Write out the formulae of each ion, including their charges
    • Al3+ SO42-
  • Balance the charges by multiplying them out:
    Al3+ x 2 = +6 and SO42- x 3 = -6; so +6 – 6 = 0
  • So the formula is Al2(SO4)3

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.

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