Isotopes of an element have :
(a) the same physical properties
(c) different number of neutrons
(b) different chemical properties
(d) different atomic numbers
(c) different number of neutrons.
Atoms of same element with same atomic number but different mass number are called isotopes of that element. For example, Hydrogen has three isotopes, protium (H11), deuterium (H21 or D21) and tritium (H31 or T31).