8.1 Chemical formula
A chemical formula represents the chemical composition of a substance by depicting
the number of atoms of different elements occurring in one fundamental "unit" of the
substance. Subscripted numbers in the formula represent the number of atoms
present if there is more than one.
In the case of pure elements, the chemical formula is the symbol itself, such as C for
carbon, Na for sodium, etc. Some elements exist as diatomic molecules (molecules
made of two atoms joined together); thus, the formula for the oxygen molecule is O2,
hydrogen is H2 and chlorine is Cl2 etc.
8.1.1 Valency
When atoms of one element combine with the atoms of another element to form
compounds, they do so in fixed numbers depending upon the capacities of the atoms to
form bonds.
The valency of an element is a measure of the combining capacity of its atom to form
chemical bonds.
The valency of an element is a whole number and varies from 1 to 8. If we know the
valencies of elements, then we can work out the chemical formulae of their compounds
by balancing the valencies of the different atoms which occur in the compound, i.e. the
total of the valencies of one set of atoms should balance the total of the valencies of
the other set.
Mg
Cl Cl
In the example above, magnesium (Mg) has a valency of 2, whereas chlorine (Cl) has a
valency of 1. To obey the rule above, the compound magnesium chloride needs one
magnesium atom and two chlorine atoms for the valencies of both to balance. Hence,
the formula is MgCl2.
Another way to consider valencies is to think of the valency number as a hook or
hooks. Each species then hooks up with another until all the hooks are connected.
TOPIC 8.1 SIMPLE FORMULAE (VALENCY)
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Mg has a valency of 2 (two hooks). Each chlorine atom has a valency of 1 (one hook).
Thus, each hook on the magnesium atom connects with the hook on each of the
chlorine atoms to give a stable compound with the formula MgCl2.
We can use valency tables to help us work out formulae of compounds.
METALS
VALENCY OF 1
VALENCY OF 2
VALENCY OF 3
Li
lithium
Mg
magnesium
Al
aluminium
Na
sodium
Ca
calcium
Fe
iron (III)
K
potassium
Ba
barium
Cu
copper (I)
Fe
iron (II)
Ag
silver
Ni
nickel
H
hydrogen
Cu
copper (II)
Sn
tin (IV)
Zn
zinc
Pb
lead
Pb
lead (II)
VALENCY OF 4
NON - METALS
VALENCY OF 1
VALENCY OF 2
VALENCY OF 3
F
fluoride
O
oxide
N
nitride
Cl
chloride
S
sulphide
PO4
phosphate
Br
bromide
SO4
sulphate
I
iodide
CO3
carbonate
OH
hydroxide
NO3
nitrate
C
carbon
NH4
ammonium
Si
silicon
TOPIC 8.1 SIMPLE FORMULAE (VALENCY)
VALENCY OF 4
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8.1.2 Finding formulae by balancing valencies
For example:
Carbon dioxide is made up of carbon and oxygen. If we look in the table above we can
see that the valency of carbon is 4 and that of oxygen is 2. In order for them to
combine, we have to balance the valency of 4 of the carbon atom with 2 atoms of
oxygen, each of valency 2. Thus, one atom of carbon will combine with two atoms of
oxygen to form the molecule CO2 (carbon dioxide). There is no need to write a 1 after
the C but other numbers must be correctly written.
C
O O
Sodium oxide is made up of sodium and oxygen We know that the valency of sodium is 1
and that of oxygen is 2. In order for them to combine, we have to balance the
valencies with 2 atoms of sodium and only 1 for the oxygen. Thus, two atoms of sodium
will combine with one atom of oxygen to form the compound Na2O (sodium oxide). By
convention we write the metal before the non-metal, just as it is written in the name.
Na Na
O
We can also work backwards:
From the chemical formula of the compound aluminium oxide, Al2O3, the valency of
aluminium can be determined. Since we know that the valency of oxygen is 2, and there
are 3 oxygen atoms, this gives a total value 6 for the valencies of the oxygen atoms,
which has to be matched by the 2 atoms of aluminium. Hence the valency of aluminium
is 3.
Al Al
TOPIC 8.1 SIMPLE FORMULAE (VALENCY)
O O O
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8.1.3 Elements with multiple valencies
Most elements have a fixed value of valency, but some exhibit two or more different
valencies and hence can form two or more different compounds with another element.
If you look up iron in the table you will find that it can have a valency of 2 or 3. So
that we are clear about which iron we are dealing with, the element's name is followed
by its valency in Roman numerals.
We can therefore have two formulae of iron chloride;
FeCl2 (iron(II) chloride) where iron has a valency of 2, or
FeCl3 (iron(III) chloride). where iron has a valency of 3.
Similarly, tin has a valency of 2 in SnCl2 (tin(II) chloride) and a valency of 4 in SnCl4
(tin(IV) chloride).
8.1.4 Polyatomic species
Upon closer inspection of the valency tables you will see that some species are listed
as a group of atoms which can combine with another element as a single unit. These
species take part as a whole (stay together) in chemical reactions and have their own
valency.
Some common examples are sulphate SO42- (valency 2), hydroxide OH- (valency 1),
carbonate CO32-(valency 2), nitrate NO3- (valency 1 ) and ammonium NH4+ (valency 1).
These have a valency equal to the charges that you see written.
Copper sulphate is made up of copper and sulphate particles. We know that the valency
of copper is 2 and that of sulphate is also 2. In order for them to combine with
balanced valencies, 1 copper must combine with 1 sulphate. Thus, the formula of copper
sulphate is CuSO4.
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8.1.5 Easy formulae by valency interchange
Notice how, whilst writing the chemical formula for a compound, the subscripts of the
constituent elements/species are obtained by interchanging the values of valencies (a
subscript of 1 is not written by convention), and if the numbers are the same they
cancel to the lowest ratio of 1:1. The following examples demonstrate this
"interchanging" of valencies.
(a) Na (valency 1) + Cl (valency 1) gives NaCl (interchange, but 1 need not be written).
(b) Al (valency 3) + O (valency 2) gives Al2O3 (interchange).
(c) Mg (valency 2) + O (valency 2) gives MgO (interchange; common factor 2 cancels
out and we write the lowest ratio).
(d) Ba (valency 2) + CO3 (valency 2) gives BaCO3.
(e) Fe (valency 3) + SO4 (valency 2) gives Fe2(SO4)3. Notice the need for brackets in
this example since we need to indicate that there are 3 of the ‘SO4’ species.
(f) NH4 (valency 1) + SO4 (valency 2) gives (NH4)2SO4. Again we need brackets since
we need to indicate that there are 2 of the ‘NH4’ species.
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