Formulas and Equations

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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
And oxygen):
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.
For example:
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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Predicting Valencies
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
Chemical Equations
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


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