Chemistry Reference
In-Depth Information
Formulae of ionic substances
On p. 39 we saw that ionic compounds contain
positive and negative ions, whose charges balance.
For example, sodium chloride contains one Na
+
ion
for every Cl
−
ion, giving rise to the formula NaCl.
This method can be used to write down formulae
which show the ratio of the number of ions present
in any ionic compound.
The formula of magnesium chloride is MgCl
2
.
This formula is arrived at by each Mg
2+
ion
combining with two Cl
−
ions, and once again the
charges balance. The size of the charge on an ion is
a measure of its
valency
or
combining power
. Na
+
has a valency of 1, but Mg
2+
has a valency of 2. Na
+
can bond (combine) with only one Cl
−
ion, whereas
Mg
2+
can bond with two Cl
−
ions.
Some elements, such as copper and iron, possess
two ions with different valencies. Copper can form
the Cu
+
ion and the Cu
2+
ion, with valencies 1 and
2 respectively. Therefore it can form two different
compounds with chlorine, CuCl and CuCl
2
. We
can also distinguish the difference by using Roman
numerals in their names: CuCl is copper(i)
i
) chloride
and CuCl
2
is copper(ii)
ii
) chloride. Similarly, iron forms
the Fe
2+
and Fe
3+
ions and so can also form two
different compounds with, for example, chlorine:
FeCl
2
(iron(ii)
ii
) chloride) and FeCl
3
(iron(ii)
iii
)
chloride).
Table 3.6 shows the valencies of a series of ions
you will normally meet in your study of chemistry.
You will notice that Table 3.6 includes groups
of atoms which have net charges. For example,
the nitrate ion is a single unit composed of one
nitrogen atom and three oxygen atoms and has one
single negative charge. The formula, therefore, of
magnesium nitrate would be Mg(NO
3
)
2
. You will
notice that the NO
3
has been placed in brackets with
a
2
outside the bracket. This indicates that there are
two nitrate ions present for every magnesium ion.
The ratio of the atoms present is therefore:
Mg (N O
3
)
2
1Mg : 2N : 6O
The charge on the element ion is often referred to as
its
oxidation state
.
Oxidation states
Each atom in an element or compound is assigned
an oxidation state to show how much it is reduced
or oxidised. The following points should be
remembered when using oxidation states.
•
The oxidation state of the free element is always 0,
for example in metals such as zinc and copper.
•
In simple ions, the oxidation state is the same as
the charge on the ion. So iodine has an oxidation
state of 0 in I
2
but an oxidation state of −1 in I
−
.
Table 3.6
Valencies (and oxidation states) of some elements (ions) and groups of atoms.
Valency (oxidation state)
1
2
3
(Li
+
)
(Mg
2
+
)
(Al
3
+
)
Metals
Lithium
Magnesium
Aluminium
(Na
+
)
(Ca
2
+
)
(Fe
3
+
)
Sodium
Calcium
Iron(
iii
)
(K
+
)
(Cu
2
+
)
Potassium
Copper(
ii
)
(Ag
+
)
(Zn
2
+
)
Silver
Zinc
(Cu
+
)
(Fe
2
+
)
Copper(
i
)
Iron(
ii
)
(Pb
2
+
)
Lead
(Ba
2
+
)
Barium
(F
−
)
(O
2
−
)
Non-metals
Fluoride
Oxide
(Cl
−
)
(S
2
−
)
Chloride
Sulfide
(Br
−
)
Bromide
(H
+
)
Hydrogen
(OH
−
)
(CO
3
2
−
)
(PO
4
3
−
)
Groups of atoms
Hydroxide
Carbonate
Phosphate
(NO
3
−
)
(SO
4
2
−
)
Nitrate
Sulfate
(NH
4
+
)
(Cr
2
O
7
2
−
)
Ammonium
Dichromate(
vi
)
(HCO
3
−
)
Hydrogencarbonate
(MnO
4
−
)
Manganate(
vii
)
