Isotopes: We all know that atoms of the same element are the same and unique in their characteristics. They all will have the same atomic number and the same mass number.

e.g. Hydrogen: Atomic number one and Mass number one.
Lithium: Atomic number three and Mass number seven.
Aluminum: Atomic number thirteen and Mass number twenty-seven.

But in nature, there are a number of cases in which atoms of the same element are found to have the same atomic number but a different mass number. Such atoms are called isotopes.

• Isotopes of hydrogen: Atomic number is 1 because the number of protons is 1 and the number of neutrons is 0.
• Deuterium with atomic number 1 and Mass number 2, so the number of protons is 1 and the number of neutrons is 1.
• Tritium with atomic number 1 and Mass number 3. So, the number of protons is 1, and the number of neutrons is 2.

In the case of chlorine

The Atomic Number of both the isotopes of chlorine is 17 but the mass number of Cl-35 is 35 and Cl-37 is 37. The number of protons in both the isotopes is 17 but the number of neutrons is 18 & 20 respectively.

Characteristics of Isotopes are:

-    They have the same atomic number or the same number of protons, and the same no. of electrons
-    They have different mass numbers and different numbers of neutrons.
-    They have the same chemical properties.

The chemical properties depend on the number of valence electrons and the number of valence electrons is the same in each isotope. So we can conclude that Isotopes of an element have:- Different physical properties, Different nuclear properties, But the same chemical properties.

Applications of Isotopes
(1)    It is used as a fuel in nuclear reactors for example isotopes of uranium.
(2)    It is used in the field of medicine.
(3)    It is also used in the process of Carbon dating to find out the age of fossils.

Calculation of Average Atomic Mass

The Mass of an atom of any natural element is taken as the average mass of all the naturally occurring atoms of that element. If an element has no isotopes then the mass of its atom would be the same as the sum of the mass of all protons and neutrons present in it. But if an element occurs in Isotopic forms, then the average atomic mass has to be calculated.

In nature, chlorine exists in two isotopes with mass numbers 35 and 37 respectively. They are found in the ratio of 3:1 i.e. 75% chlorine is of mass number 35 and 25 % chlorine is of mass number 37.

Hence, Average atomic mass =35 x (75/100) + 37 x (25/100)
= ( (35 x 75) + (37 x 25) )/100
= 35.5 AMU

This does not mean that anyone atom of chlorine has a fractional mass of 35.5 AMU; rather it means that if you take a certain amount of chlorine, it will contain both isotopes of chlorine and the average mass would be equal to 35.5 AMU.

Read More: Structure of an Atom: Electrons, Protons, Neutrons | Sub Atomic Particles