Protium, deuterium, and tritium are isotopes of the hydrogen element. Isotopes are different forms of the same element that are different from each other according to the number of neutrons they have in their nuclei.

Why is deuterium not in periodic table?

It is not on the periodic table! Deuterium is simply Hydrogen with one extra neutron (i.e., 1 proton, but the same atomic weight as He). Its natural abundance (which is virtually nothing) is factored into the average atomic weight of H which as you notice is NOT exactly 1 amu).

Is tritium an element?

Tritium is a radioactive isotope of the element hydrogen. What are the properties of tritium? Tritium is radioactive and has a half-life of about 12.5 years, which means that half of the radioactive atoms will decay naturally in that time.