Page 23 - Chemistry ICSE Class IX

The Language of Chemistry 11
ƒ Hydrogen sulphide — Hydrogen sulphide contains hydrogen and
sulphur. Sulphur is more electronegative than hydrogen. Then by
following the steps given above, we can write

Step 1. H S
1+
Step 2. H o o S 2–
Step 3. H S or H S
2 1
2
So, the molecular formula of hydrogen sulphide is H S.
2
ƒ Carbon tetrachloride — Carbon tetrachloride contains carbon and
chlorine. Chlorine is more electronegative than carbon. So
Step 1. C Cl
4+
Step 2. C o o Cl 1–
Step 3. C Cl 4 or CCl 4
1
So, the molecular formula of carbon tetrachloride is CCl .
4

Writing the Formulae of Ionic Compounds
&Žƌ Ă ĐŽŵƉŽƵŶĚ ĐŽŶƐŝƐƟŶŐ ŽĨ Ă
ŵĞƚĂů ĂŶĚ Ă ŶŽŶŵĞƚĂů͕ ƚŚĞ ŶĂŵĞ Žƌ
How to write the chemical formulae of ionic compounds ƚŚĞ ƐLJŵďŽů ŽĨ ƚŚĞ ŵĞƚĂů ŝƐ ǁƌŝƩĞŶ
The ionic (or electrovalent) compounds contain ions. These ions are arranged ĮƌƐƚ͘ dŚŝƐ ŝƐ ƚŚĞŶ ĨŽůůŽǁĞĚ ďLJ ƚŚĞ
in such a way that a cation has an anion as its nearest neighbour and vice- ƐLJŵďŽů ŽĨ ƚŚĞ ŶŽŶŵĞƚĂů͘
versa. There is no discrete molecule of an ionic compound. Therefore, it is
not correct to assign a molecular formula to an ionic compound. Instead,
an ionic compound is described by a formula which describes a simple
atomic ratio of the elements present in it.
The formula which describes the simplest atomic ratio of the
elements present in a compound is called its stoichiometric formula (or
chemical formulae).
For example, sodium chloride is an ionic compound in which sodium dŚĞ ƐƚŽŝĐŚŝŽŵĞƚƌŝĐ ĨŽƌŵƵůĂ ŽĨ
and chlorine are present in 1 : 1 ratio. So, the stoichiometric (or chemical) ĂŶ ŝŽŶŝĐ ĐŽŵƉŽƵŶĚ ĐĂŶ ďĞ ĞĂƐŝůLJ
–
+
formula of sodium chloride is Na Cl (or NaCl). ǁƌŝƩĞŶ ŝĨ ƚŚĞ ĐŚĂƌŐĞƐ ŽŶ ƚŚĞ ŝŽŶƐ
ĨŽƌŵŝŶŐ ƚŚĞ ĐŽŵƉŽƵŶĚ ĂƌĞ ŬŶŽǁŶ͘
To write the stoichiometric (or chemical) formula of an ionic compound,
proceed as follows:
ƒ Write the symbol of the cation showing the charge number on it.
ƒ Write the symbol of the anion showing the charge number on it, on
the right hand side of the cation.
ƒ Now, write the charge number (valency) of the cation at the bottom
right of the anion and the charge number of the anion at the bottom of
the cation. Thus, the symbol of cation is subscripted with the charge
number of the anion and the anion is subscripted with the charge
number of the cation. This is called the criss-crossing of valencies.
ƒ If a compound contains polyatomic ions, then the formula of the
polyatomic ion is enclosed within brackets before criss-crossing the
valencies.
ƒ If these subscripts are 1, these are not written in the nal formulae.
Otherwise these subscripts are reduced to the lowest possible integers
by dividing each by the highest common factor.
This method of writing the chemical formulae for ionic compounds is
illustrated below.
2–
3+
1. Aluminium sulphate. Aluminium sulphate contains Al and SO
4
ions. The formula for aluminium sulphate can be obtained as follows: