Page 44 - Understanding NCERT Science 09

The rules that you have to follow while writing 3. Formula of carbon tetrachloride
a chemical formula are as follows:
• the valencies or charges on the ion
must balance.
• when a compound consists of a metal and
a non-metal, the name or symbol of the
metal is written first. For example: calcium
oxide (CaO), sodium chloride (NaCl), iron
sulphide (FeS), copper oxide (CuO), etc.,
where oxygen, chlorine, sulphur are non- For magnesium chloride, we write the
metals and are written on the right, symbol of cation (Mg ) first followed by the
2+
whereas calcium, sodium, iron and symbol of anion (Cl ). Then their charges are
-
copper are metals, and are written on criss-crossed to get the formula.
the left.
• in compounds formed with polyatomic ions, 4. Formula of magnesium chloride
the number of ions present in the
compound is indicated by enclosing the
formula of ion in a bracket and writing the
number of ions outside the bracket. For
example, Mg (OH) . In case the number of
2
polyatomic ion is one, the bracket is not Formula : MgCl 2
required. For example, NaOH.
Thus, in magnesium chloride, there are
3.4.1 FORMULAE OF SIMPLE COMPOUNDS -
two chloride ions (Cl ) for each magnesium
The simplest compounds, which are made up ion (Mg ). The positive and negative charges
2+

of two different elements are called binary must balance each other and the overall
compounds. Valencies of some ions are given structure must be neutral. Note that in the
in Table 3.6. You can use these to write
formulae for compounds. formula, the charges on the ions are
not indicated.
While writing the chemical formulae for
compounds, we write the constituent elements
and their valencies as shown below. Then we Some more examples
must crossover the valencies of the (a) Formula for aluminium oxide:
combining atoms.

Examples
1. Formula of hydrogen chloride
Formula : Al O
2 3

(b) Formula for calcium oxide:
Formula of the compound would be HCl.

2. Formula of hydrogen sulphide

Here, the valencies of the two elements
are the same. You may arrive at the formula
Ca O . But we simplify the formula as CaO.
2 2

ATOMS AND MOLECULES 33

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