Page 56 - Science Class 07

Atomicity is called the chemical formula of water.
Atomicity is the number of atoms present in a
molecule.
For example, Oxygen exists as a diatomic
molecule which means that a molecule of an
oxygen contains two atoms hence its atomicity
is 2.
Similarly, a phosphorus molecule contains 4
atoms, a sulphur molecule contains 8 sulphur
atoms. Hence, their atomicity is 4 and 8
respec vely.
For molecule containing more than one type
of atoms, simply count the number of each
atom and that would be its atomicity.
For example, a molecule of sulphuric acid
(H SO ) consists of 2 hydrogen atoms, 1 sulphur
2 4
atom and 4 oxygen atoms. Hence, its atomicity
is 2+1+4=7.
v Some mes, one molecule of an element
contains more than two atoms. These
elements are called Polyatomic, e.g.,
chemical formula of Phosphorus molecule
is P and its atomicity is 4.
4 A B + C, In it, a single substances breaks up
So, when the atoms of different elements
combine chemically in whole numbers and in a
definite ra o, the molecule of a new substance
is formed.
Let us have a few examples:
v When two atoms of hydrogen chemically
combines with one atom of oxygen, a
molecule of water is formed.
v When one atom of carbon chemically
combines with two atoms of oxygen, a
molecule of carbon dioxide is formed.
One molecule of water is wri en as H O
2
using symbols of hydrogen and oxygen. This
56
Similarly, the chemical formula of carbon
dioxide is wri en as CO . So, a chemical
2
formula of a compound is a set of element
symbols wri en along with their atoms that
make up its one molecule. Chemical
formula of two or more molecules of an
element or a compound, some mes, more
than one molecule of an element is needed
to carry out chemical reac ons. In such
case, to write chemical formula, the
number of the molecules are represented
by the prefix on the le hand side of the
chemical formula, e.g, to represent 2
molecules of hydrogen we write 2H , here,
2
H is chemical formula of hydrogen and 2H
2 2
represents two molecules of hydrogen.
Similarly, we can also represent 2 molecules
of Hydrogen Chloride (HCl), to represent its
two molecules we write 2HCl.
Types of Chemical Reac ons
A B. In it, a single substance undergoes a
chemical reac on to become a new substance;
as : A piece of paper burns into ash.
into two new substances; as : water breaks up
into oxygen and hydrogen.
A + B C. In it, two substances react to form a
new substance; as : Oxygen and hydrogen react
to form water.
A + B C + D. In it, two substances react to
form two new substances ; as : Iron as sulphuric
acid reacts to form iron sulphate and hydrogen.
Clearly, if we know the names of elements with
the number of their atoms that make up the
molecule of a compound, we can write its
formula. On the next page, there is a list of the
formulae of some chemical compounds.
Science-7