Page 56 - Understanding NCERT Science 09

Many elements consist of a mixture of Applications
isotopes. Each isotope of an element is a pure
Since the chemical properties of all the
substance. The chemical properties of isotopes of an element are the same,
isotopes are similar but their physical normally we are not concerned about
properties are different.
taking a mixture. But some isotopes have
Chlorine occurs in nature in two isotopic special properties which find them useful
forms, with masses 35 u and 37 u in the ratio in various fields. Some of them are :
of 3:1. Obviously, the question arises: what (i) An isotope of uranium is used as a fuel
should we take as the mass of chlorine atom? in nuclear reactors.
Let us find out. (ii) An isotope of cobalt is used in the
The average atomic mass of chlorine atom, treatment of cancer.
on the basis of above data, will be (iii) An isotope of iodine is used in the
treatment of goitre.
  75 25 
  35 × + 37 × 
  100 100 4.6.1 ISOBARS
Let us consider two elements — calcium,
105 37  142  atomic number 20, and argon, atomic
=  +  = = 35 5. u
 4 4  4   number 18. The number of protons in these
atoms is different, but the mass number of
both these elements is 40. That is, the total
The mass of an atom of any natural element
number of nucleons is the same in the atoms
is taken as the average mass of all the naturally
of this pair of elements. Atoms of different
occuring atoms of that element. If an element
elements with different atomic numbers, which
has no isotopes, then the mass of its atom
have the same mass number, are known as
would be the same as the sum of protons and
isobars.
neutrons in it. But if an element occurs in
isotopic forms, then we have to know the uestions
percentage of each isotopic form and then the
average mass is calculated. 1. For the symbol H,D and T
This does not mean that any one atom of tabulate three sub-atomic
chlorine has a fractional mass of 35.5 u. It Q particles found in each of them.
means that if you take a certain amount of 2. Write the electronic
chlorine, it will contain both isotopes of configuration of any one pair of
chlorine and the average mass is 35.5 u. isotopes and isobars.

What

you have

learnt

• Credit for the discovery of electron and proton goes to J.J.
Thomson and E.Goldstein, respectively.
• J.J. Thomson proposed that electrons are embedded in a
positive sphere.

STRUCTURE OF THE ATOM 45

Rationalised 2023-24