Formulae (CIE IGCSE Chemistry)

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

Chemical symbols of elements table

Element	Symbol	Element	Symbol
Lithium	Li	Calcium	Ca
Chromium	Cr	Gold	Au
Aluminium	Al	Sulfur	S

 

• 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

Substance	Molecular formula	Made from
Hydrogen	H2	2 hydrogen atoms
Chlorine	Cl2	2 chlorine atoms
Water	H2O	2 hydrogen atoms 1 oxygen atom
Methane	CH4	1 carbon atom 4 hydrogen atoms
Ammonia	NH3	1 nitrogen atom 3 hydrogen atoms
Sulfuric acid	H2SO4	2 hydrogen atoms 1 sulfur atom 4 oxygen atoms

• 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 elemetns and simple compounds: