Valences of each elemental group

Group Valency
I 1
II 2
IV +/- 4
V -3
VI -2
VII -1


Valences of common ions

Name Formula Valency
Iron (II) Fe2+ +2
Copper (II) Cu2+ +2
Ammonium NH4+ +1
Nitrate NO3 -1
Hydroxide OH -1
Carbonate CO32- -2
Sulphate SO42- -2
Silicate SiO32- -2
Phosphate PO43- -3


Word equations

  • Balancing equations: A chemical equation is balanced when there are equal number of atoms and charges on both sides of the equation
  • State symbols:
    • (s) = solid
    • (l) = liquid
    • (g) = gas
    • (aq) = aqueous solution
  • Names of compounds
    • Compound ending with -ide only contain two different elements
    • Compound ending with -ate contain oxygen


  • Relative atomic mass (Ar): mass of one atom of an element relative to one twelfth of the mass of one atom of Carbon-12
  • Relative molecular mass (Mr): sum of relative atomic masses of all the atoms in one molecule of the compound

The Mole Concept

  • A mole of a substance is the amount that contains the same number of units as the number of carbon atoms in 12 grams of carbon-12
  • A mole is the Ar or Mr expressed in grams e.g. 1 mole of Carbon-12 is equal to 12 grams.
  • It is equal to 6.02 × 1023 particles, this number is called Avogadro’s constant.


Number of Moles

Number of Moles = mass/ molar mass

Moles in Gases

Volume = No. of Moles×24dm3 at r.t.p (room temperature and pressure)


concentration = (no. of moles) / volume

  • Moles per dm3
    • 1mol/dm3 = 1M
  • Grams per dm3 or g/dm3

Molecular Formulae

  • It shows the actual number of atoms in one molecule of a substance.

Empirical Formulae

  • This is the simplest ratio of the atoms in a compound
  • For example:
    • Molecular formula of ethane= C2H6
    • Empirical formula of ethane = CH3
  • To find out the empirical formula you:
    • Make the percent ratio into the simplest whole number ratio (NOTE: if given %s, use them as grams)
    • Divide the coefficients of each element symbol by the lowest coefficient


  • Percentage purity = (mass of product / mass of compound) x 100
  • Percentage yield = actual mass obtained / calculated mass



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