Formulae (Cambridge (CIE) O Level Chemistry): Revision Note

Molecular Formulae

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

  • E.g. 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

  • E.g. 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

  • E.g. 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                                                                             

  

The ammonia molecule consists of a central nitrogen atom bonded to three hydrogen atoms

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 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)

• Structural formula (simplified)

CH₃CH₂CH₂CH₃

• Molecular formula

C₄H₁₀

• Empirical formula

C₂H₅

Deducing formulae by Combining power(valency)

• The concept of valency is used to deduce the formulae of compounds (either molecular compounds or ionic compounds)

• Valency or combining power tells you how many bonds an atom can make with another atom or how many electrons its atoms lose, gain or share, to form a compound

  • E.g. 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: