Question

Explain the following:

An element has atoms with different mass numbers.

Answer 1

In nature, atoms of the same element are found to exist with different mass numbers.

• This phenomenon is called Isotopy and the atoms of the same element which have the same atomic number but different mass numbers are called as isotopes.

The atoms of these elements differ in the number of neutrons due to which their mass number varies as the mass number is the sum of the number of protons and neutrons.

• Mass number= Number of protons + Number of neutrons

Example: Chlorine (Cl). Some atoms of Chlorine have a mass number of 35 $\left({}_{17}{}^{35}\mathrm{Cl}\right)$ whereas others have a mass number of 37 $\left({}_{17}{}^{37}\mathrm{Cl}\right)$.

Number of protons in both of these type of atoms= 17

Number of neutrons= Mass number- Number of protons

• Number of neutrons in $\left({}_{17}{}^{35}\mathrm{Cl}\right)$= 35- 17= 18
• Number of neutrons in $\left({}_{17}{}^{37}\mathrm{Cl}\right)$= 37- 17= 20