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FORMULAS AND EQUATIONS
Chemists use formulas and equations to save writing the full names of substances.
The valency of an atom can be used to work out the formula of a compound.
1 2 3 4
Na Sodium Mg Magnesium Al Aluminium C Carbon
K Potassium Ca Calcium Fe Iron (lll) Si Silicon
H Hydrogen Cu Copper N Nitrogen
F Fluorine Zn Zinc
Cl Chlorine Fe Iron (ll)
Br Bromine O Oxygen
I Iodine S Sulphur
For compounds made of molecules, the valency is the number of bonds each atom forms.
Water is a compound made up of hydrogen and oxygen. Oxygen has a valency of two and so forms two
bonds. Hydrogen has a valency of one and so forms one bond. This means that two hydrogen atoms must
join to one oxygen atom, hence the formula being HO.
Water (contains hydrogen H O Formula: HO
valencies: 1 2
Although many compounds are no made of molecules (they contain particles called ions), the formulas of
most compounds can be worked out using the cross-over method.
Some atoms have two or more different valencies. For example, iron can have a valency of two or three,
and so its compounds have the roman numerals (ll) or (lll) in their names to define the valency of the iron.
Iron (ll) chloride (contains Fe Cl Formula: FeCl
Iron (ll) and chlorine):
valencies: 2 1
Iron (lll) chloride (contains Fe Cl Formula: FeCl
Iron (lll) and chlorine):
valencies: 3 1
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There is a link between the valencies of an atom and its position in the periodic table.
Group 1 2 3 4 5 6 7
Valency 1 2 3 4 3 2 1
We often use word equations to describe what happens in a chemical reaction. For example, the reaction
of methane with oxygen is:
Methane + oxygen = carbon dioxide + water
However we often write equations with formulas rather than words.…read more