Formulae (Cambridge International IGCSE Chemistry)

Molecular formulae

Spec point — 3.1.1 · State the formulae of the elements and compounds named in the subject content
Define the molecular formula of a compound as the number and type of different atoms in one molecule Deduce the formula of a simple compound from the relative numbers of atoms present in a model or a diagrammatic representation

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

Worked Example What is the molecular formula of the following compound?

Answer:

• The molecule contains:

  • 4 carbon atoms
  • 10 hydrogen atoms

• Therefore, the molecular formula is C₄H₁₀