Explain by illustrating the types of atoms. 

The measurement of atomic masses reveals the existence of different types of atoms of the same element which exhibit the same chemical properties but differ in mass such type of atoms are called isotopes.

"Isotope are atoms that have the same number of $Z$ but different number of $A$ and $N$ ".

Isotopes occur in the same place in the periodic table of elements.

The relative abundance of different isotopes differs from elements to element.

For example :

$(1)$ Chlorine has two isotopes having masses $34.98 \mathrm{u}$ and $36.98 \mathrm{u}$ which are nearly integral multiples of the mass of a hydrogen atom. The relative abundances of these isotopes are $75.4$ and $24.6 \%$. Thus, the average mass of a chlorine atom is obtained by the weighted average of the masses of the two isotopes.

Average mass of chlorine,

$=\frac{75.4 \times 34.98+24.6 \times 36.98}{100}$

$=35.47 \mathrm{u}$

$(2)$ The lightest element hydrogen has three isotopes.

Mass of hydrogen atom $=1.0078 \mathrm{u}$,

Mass of deuterium atom $=2.0141 \mathrm{u}$

Mass of tritium atom $=3.0160 \mathrm{u}$

The nucleus of hydrogen has a relative abundance of $99.985 \%$ and its nucleus is called proton.

In atom of hydrogen there is no neutron but have only one proton.

Mass of proton $m_{p}=1.0078 \times 99.985 \%$

$=1.007648 \mathrm{u}$ $\approx 1.00727 \mathrm{u} \text { is taken }$

$\therefore \text { Mass of proton }$ $m_{p}=1.00727 \times 1.660539 \times 10^{-27} \mathrm{~kg}$ $m_{p}=1.67262 \times 10^{-27} \mathrm{~kg} \text { is taken }$

The value of mass of this proton,

$=$ Mass of hydrogen atom $-$ mass of electron

$=(1.00783 \mathrm{u}-0.00055 \mathrm{u})$

$=1.00728 \mathrm{u}$

Isotopes of deuterium and tritium are being unstable, they do not occur naturally and are

produced artificially in laboratories.

Hydrogen, deuterium and tritium are isotopes and do not have neutrons in their nucleus but

have only one proton, so their ratio of masses is $1: 2: 3$.