Deuterium is an isotope of which element?

The story behind the answer

Hydrogen. Deuterium is the “heavy” isotope of hydrogen, written ²H or D, with one proton and one neutron in its nucleus. Ordinary hydrogen (protium, ¹H) has one proton and no neutrons; tritium (³H) has two neutrons. Isotopes have the same number of protons (so the same element) but differ in mass.

Discovered by Harold Urey in 1931, deuterium makes up about 0.015% of natural hydrogen and forms heavy water (D₂O), used as a neutron moderator in some nuclear reactors and in N