Elements are chemically pure substances. This means that, they consist of only one type of atoms in them. An atom consists of protons and neutrons at the nucleus and electrons revolving around the nucleus. Number of protons and electrons are always equal. Neutrons are charge-less subatomic particles equal in mass with the protons. Chemical properties of any element are governed by the number of electrons present in it. All the isotopes have equal number of electrons in their orbits; hence, all isotopes have same chemical properties.
Isotopes of an element differ from one another with respect to the number of neutrons present in their atom. An isotope may have more number of neutrons than other. Since isotopes have different number of neutrons, their total of proton and neutron (mass number) also differs.