Page 103 - ICSE Chemistry 8

Take a Break!

Fill in the blanks.
1. The ______________ of an atom is a whole number that varies from 1 to 8.

2. ______________ is the outermost shell of the atom.
3. A ______________ is formed when an atom or a group of atoms loses electron(s) by developing a
posi ve charge.

4. A ______________ refers to a group of diff erent atoms present as a single unit having a posi ve or
nega ve charge on it.
5. The chemical formula of ammonium is ______________ and its valency is ______________.

Chemical Formulae of Compounds

Compound are formed by the chemical combina on of two or more elements. A chemical formulae or a
molecular formula is the representa on of a molecule of a compound symbolically.

It is wri en by wri ng the elements involved and then the number of atoms of each element involved in
the compound. For example, methane is a compound formed by four atoms of hydrogen and one atom
of carbon. So the formula of one molecule of methane is CH .
4
The chemical formula of a compound is derived in the following way.

Step 1: Symbols of the elements forming the compound are wri en.

Step 2: Valency of each element is wri en below the symbols.

Step 3: Valencies are interchanged and are wri en as subscripts next to the symbols. Subscript is not
required if the valency is 1.

Step 4: Valencies are divided by a common number to simplify the formula. However, if it is not possible,
the valencies are wri en as they are.
Let’s understand the representa on of compounds with the help of examples.

Example 1: A molecule of water is formed by combining the atoms of the elements hydrogen and oxygen.

Step 1: Write symbols of the elements hydrogen and oxygen.
Hydrogen: H Oxygen: O
Thinking Fountain
Step 2: Write valencies of the elements. What is the role of valency in
wri ng a chemical formula of
H O
a compound?
1 –2
Step 3: Interchange the valencies and write as subscripts next to the elements.

H O
2
Step 4: Since no further division of valency is possible, therefore the chemical formula of water is H O.
2

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