why do isotopes show similar chemical properties ?

Isotopes are defined as atoms of same element having the same atomic number, but different mass numbers. These atoms contain an equal number of protons and electrons, but a different number of neutrons. For example, C- 12, C-13 and C-14 are isotopes of carbon which have the same number of electrons and protons, but the number of neutrons are different. 

The three isotopes of hydrogen are protium (), deuterium () and tritium (  ). These are the three naturally occurring isotopes of hydrogen. The atomic number of each isotope is 1, but their mass numbers are 1, 2, and 3 respectively.

Since it is the valence electrons which participate in bond formation, therefore chemical properties of elements depend on number of electrons and thereby electronic configuration. Hence, isotopes of an element have similar chemical properties since they have the same number of electrons, and therefore similar electronic configuration.