A-LEVEL OCR ChEMISTRY NOTES 
Compounds, formulae and equations

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Ionic Compounds
  • An ion is an atom or molecule with a net charge formed through the gain or loss of electrons
  • A cation is formed from the loss of electrons
  • An anion is formed from the gain of electrons
  • Ionic compounds are composed of oppositely charged ions. The overall charge is zero as the ionic charges balance.
  • e.g. write the formula of sodium carbonate
We need 2 x Na+ to balance the -2 charge of CO3^2-
The formula is Na2CO3​

Balanced Equations
  • When a chemical reaction occurs, no atoms are created or destroyed. The atoms in the reactants rearrange to form the products.
  • In a balanced equation, there is the same number of atoms of each element in both the reactants and products.
  • State symbols are written after every species to indicate the physical state
    • Solid (s)
    • Liquid (l)
    • Gaseous (g)
    • Aqueous (aq) - dissolved in water
  • Ionic equations can be written for any reaction involving ions in solution, where only the reacting ions and the products they form are included.
  • The net ionic equation shows only the ions directly involved in the reaction (removing spectator ions). e.g. 
NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)
Na+ (aq) + Cl- (aq) + Ag+ (aq) + NO3- (aq)→ AgCl (s) + Na+ (aq) + NO3- (aq)

Net ionic equation:
Cl- (aq) + Ag+ (aq) → AgCl (s)