Which isotope of hydrogen is present in heavy water?

Isotopes of an element share the same atomic number (Z) — the same number of protons, so they undergo the same types of chemical reactions — but differ in mass number (A), because they carry a different number of neutrons; this mainly changes physical properties such as density (hydrogen's isotopes, with their large relative mass difference, can also show small rate/equilibrium differences called isotope effects, but the core chemistry stays the same).

Hydrogen has three natural isotopes, all with atomic number 1 (one proton): Protium (1H, no neutron, mass number 1), Deuterium (2H, also written D, one neutron, mass number 2), and Tritium (3H, two neutrons, mass number 3, radioactive). Heavy water is defined as D2O — ordinary water (H2O) in which both hydrogen atoms are replaced by Deuterium. The extra neutron in every Deuterium atom raises the molar mass from about 18 g/mol (H2O) to about 20 g/mol (D2O), so the isotope whose presence gives water this measurable extra mass — the isotope present in heavy water — is Deuterium.

Cross-check against the other isotopes:

• Protium (1H, mass number 1, no neutron) is the ordinary hydrogen isotope found in normal water; carrying no extra neutron, it adds no extra mass, so it cannot be the isotope that makes water "heavy".

• Tritium (3H, mass number 3, two neutrons) is radioactive; water made with Tritium is called tritiated (or "super-heavy") water, a distinct substance from heavy water.

• Protium and Tritium together still do not describe a single one-neutron isotope; heavy water is built from Deuterium alone.

So Deuterium (2H) — the isotope of hydrogen with one proton and one neutron — is the isotope present in heavy water.