An isotope has more or less number of neutrons in the nucleus.
The nucleus of an atom is made up of two fundamental particles, the protons and the neutrons. Protons are positively charged particles in an atom. Neutrons do not carry any charges whatsoever.
Isotopy is the existence of two or more atoms of the same element having the same atomic number but different mass numbers due to the differences in the number of neutrons.
Isotopes differ in masses due to the number of neutrons they contain.
