Molecular formulae

Elements are often represented using their chemical symbol from the Periodic 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 

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

Diagrammatic representation of chemicals

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

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