Page 42 - Understanding NCERT Science 09

3.3.1 MOLECULES OF ELEMENTS Table 3.4 : Molecules of some

The molecules of an element are constituted compounds
by the same type of atoms. Molecules of many Compound Combining Ratio
elements, such as argon (Ar), helium (He) etc. Elements by
are made up of only one atom of that element. Mass
But this is not the case with most of the non- Water (H O) Hydrogen, Oxygen 1:8
2
metals. For example, a molecule of oxygen Ammonia (NH ) Nitrogen, Hydrogen 14:3
3
consists of two atoms of oxygen and hence it
Carbon
is known as a diatomic molecule, O . If 3
2 dioxide (CO ) Carbon, Oxygen 3:8
atoms of oxygen unite into a molecule, instead 2
of the usual 2, we get ozone, O . The number
3
of atoms constituting a molecule is known as Activity ______________ 3.2
its atomicity. • Refer to Table 3.4 for ratio by mass of
Metals and some other elements, such as atoms present in molecules and Table
carbon, do not have a simple structure but 3.2 for atomic masses of elements. Find
the ratio by number of the atoms of
consist of a very large and indefinite number
elements in the molecules of
of atoms bonded together. compounds given in Table 3.4.
Let us look at the atomicity of some • The ratio by number of atoms for a
non-metals. water molecule can be found as follows:

Element Ratio Atomic Mass Simplest
Table 3.3 : Atomicity of some by mass ratio/ ratio
elements mass (u) atomic
mass
Type of Name Atomicity
Element 1
H 1 1 =1 2
1
Non-Metal Argon Monoatomic
8 1
Helium Monoatomic O 8 16 = 1
16 2
Oxygen Diatomic
• Thus, the ratio by number of atoms for
Hydrogen Diatomic
water is H:O = 2:1.
Nitrogen Diatomic
Chlorine Diatomic 3.3.3 WHAT IS AN ION?

Phosphorus Tetra-atomic Compounds composed of metals and non-
Sulphur Poly-atomic metals contain charged species. The charged
species are known as ions. Ions may consist
of a single charged atom or a group of atoms
that have a net charge on them. An ion can be
negatively or positively charged. A negatively
3.3.2 MOLECULES OF COMPOUNDS
charged ion is called an ‘anion’ and the
Atoms of different elements join together positively charged ion, a ‘cation’. Take, for
in definite proportions to form molecules example, sodium chloride (NaCl). Its
of compounds. Few examples are given in constituent particles are positively charged
+
Table 3.4. sodium ions (Na ) and negatively charged
ATOMS AND MOLECULES 31

Rationalised 2023-24