Formulae (Cambridge (CIE) IGCSE Chemistry)

Molecular formulae

• Elements are often represented using their chemical symbol from the Periodic Table

Chemical symbols of elements table

• Seven elements exist as diatomic molecules, which means that they are molecules made of two atoms:

  1. Hydrogen, H₂ 

  2. Nitrogen, N₂ 

  3. Oxygen, O₂ 

  4. Fluorine, F₂ 

  5. Chlorine, Cl₂ 

  6. Bromine, Br₂ 

  7. Iodine, I₂ 

Periodic Table identifying the 7 diatomic elements

The highlighted elements all exist as diatomic molecules with the formula X₂

• These 7 elements are also classed as simple molecules

• Atoms combine together in fixed ratios that will give them full outer shells of electrons

  • When this happens, a molecule is formed

• The molecular formula of a molecule shows:

  • The type of atoms involved, given by the chemical symbol
    AND

  • The number of atoms, given by the subscript (little) number after a chemical symbol

Examples of molecular formulae

• The table also shows that the molecular formula can be deduced from the relative number of atoms present

  • E.g. Ammonia contains 3 atoms of hydrogen and 1 atom of nitrogen, which means its molecular formula is NH₃

Diagrammatic representation of chemicals

• Diagrams or models can be used to represent and/or deduce the molecular formula of elements and simple compounds:

When simple molecules are represented using coloured atoms, a key is used to show the colours for each type of atom

• Using the water examples, it is important to know that these representations also show the arrangement of the atoms in the molecule

  • Water, H₂O

    • The hydrogen atoms are on either side of the oxygen atom

    • It does not have two hydrogen atoms and an oxygen atom all joined together in a row