Isotopes are chemical elements with same atomic number (Z), but different mass number (different number of neutrons).
Hydrogen is an element with atomic number 1. It has three isotopes:
1) protium (₁¹H), with mass number 1 (no neutrons).
2) deuterium (₁²H), with mass number 2 (one neutrons, n° = 2 -1, n° = 1).
3) tritium (₁³H), with mass number 3 (two neutrons, n° = 3 -1, n° = 2).
