Terrestrial hydrogen contains deuterium mostly in the form of _______.

Isotopes

• Elements with the same atomic number but a different mass number are called isotopes.
• The isotopes are different because of the different number of neutrons present in them.

Isotopes of hydrogen

• There are three isotopes of hydrogen; protium(${}_1{}^1\mathrm{H}$), deuterium(${}_2{}^1\mathrm{H}$), and tritium(${}_3{}^1\mathrm{H}$).
• Terrestrial hydrogen contains 0.0156% of deuterium mainly in the form of HD.

Hence terrestrial hydrogen contains deuterium mostly in the form of HD.