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CGB IGCSE Chemistry
Writing Chemical Formulas
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Ions

Positive Ions
 
Lithium = Li +
Sodium= Na +
Potassium= K +
Silver= Ag +
Copper(I)= Cu +
Ammonium= NH4 +
Hydrogen= H +
 
Berillium= Be 2+
Magnesium= Mg 2+
Calcium= Ca 2+
Iron(II)= Fe 2+
Copper(II)= Cu 2+
Zinc= Zn 2+
 
Aluminum= Al 3+
Iron(III)= Fe 3+
 
 

Negative Ions
 
Hydride= H -
Chloride= Cl -
Bromide= Br -
Iodide= I -
Fluoride= F -
Hydroxide= OH -
Nitrate= NO3 -
Hydrogen carbonate = HCO3 -
 
Oxide= O 2-
Sulphide= S 2-
Sulphate= SO4 2-
Carbonate= CO3 2-
 
Nitride= N 3-
Phosphate= PO4 3-
 

To write the formula first look at the name of the compound.
     For example magnesium chloride.
 
Magnesium chloride has 2 ions - the magnesium ion and the chloride ion. 
 
Determine the oxidation numbers (charges) on these 2 ions.
    Magnesium = Mg 2+
    Chloride = Cl -
 
The oxidation number (charge) of a compound must be neutral.  To determine the oxidation number of a compound add the oxidation numbers of the ions.
 
oxidation number of compound = (# of ions)(oxidation #) + (# of ions)(oxidation #)
 
So if the formula of magnesium chloride was MgCl the oxidation number of the compound would be:
           =(1)(+2) + (1)(-1)
           = +2 + -1
           = 2-1
           = 1
          therefore the formula cannot be MgCl
 
If the formula of magnesium chloride was MgCl2 the oxidation number of the compound would be:
           =(1)(+2) + (2)(-1)
           = +2 + -2
           = 2-2
           = 0
           therefore the correct formula for magnesium chloride is MgCl2
 
An easy way to determine formulas is to switch the oxidation numbers.  Remember a "1" is never written in the formula.