Atomic Number and Mass Number, Isotopes and Isobars

11th Chemistry NCERT notes

Atomic Number and Mass Number

Atomic Number (Z):

The number of protons present in the nucleus is equal to the atomic number (Moseley 1913).

Atomic Number (Z)= Number of protons in the nucleus of an atom = Number of electrons in a neutral atom


  • The number of protons in the hydrogen nucleus is 1, in sodium atom, it is 11, therefore, their atomic number are 1 and 11.
  • In order to keep the electrical neutrality, the number of electrons in an atom is equal to the number of protons (atomic number, Z).
  • For example, the number of electrons in a hydrogen atom and sodium atom are 1 and 11 respectively.

Mass Number (A):

Sum of the number of protons and neutrons present in the nucleus.

Mass Number (A) = Number of protons (p) + Number of neutrons (n).


Atoms with an identical atomic number but a different atomic mass number are known as Isotopes.

Isotopes of Hydrogen:

Isotopes of Hydrogen

These three isotopes as shown in the figure below:

Isotopes of hydrogen

Isotopes of Chlorine:

These are two isotopes of chlorine with mass number 35 and 37.

Isotopes of chlorine

The two isotopes differ in their number of neutrons, having 18 and 20 neutrons, respectively.


  • Isobars are the atoms with the same mass number but the different atomic number.

For example,


  • Each of these has the same mass number but a different atomic number.



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