Hydrogen has three isotopes: protium ($$^1$$H), deuterium ($$^2$$H or D) and tritium ($$^3$$H or T). They have nearly same chemical properties but different physical properties. They differ in

Hydrogen has three isotopes: Protium ($$^1$$H) with mass number 1 (1 proton, 0 neutrons), Deuterium ($$^2$$H or D) with mass number 2 (1 proton, 1 neutron), and Tritium ($$^3$$H or T) with mass number 3 (1 proton, 2 neutrons).

Since they are isotopes of the same element (hydrogen), they have the same number of protons (1 each) — so Option A is incorrect; the same atomic number ($$Z = 1$$) — so Option B is incorrect; and the same electronic configuration ($$1s^1$$) — so Option C is incorrect.

They differ in the number of neutrons, which means they have different mass numbers and therefore different atomic masses.

This is why isotopes have nearly the same chemical properties (determined by electronic configuration) but different physical properties (affected by mass).

Hence, the correct answer is Option D (Atomic mass).