The number of nucleons in the isotope of an atom ${}_{\mathrm{z}}\mathrm{X}^{\mathrm{m}}$ are

The isotope an atom is ${}_{\mathrm{z}}\mathrm{X}^{\mathrm{m}}$

Here, the letter; ‘Z’ indicates atomic number of the element X. 

Z =atomic number  

The letter ‘m’ indicates mass number of the element X. 

The number of nucleons in the isotope of an atom is equal to the sum of number of protons and number of neutrons. 

No .of nucleons = p + n=m.