Question

i know about three isotopes of hydrogen-

protium , deuterium and tritium

but protium have atomic mass as 1 u.

then why it is called a isotope.

is has mass equal to simple hydrogen atom

Answer 1

Isotope: A given element is identified by the number of protons in its nucleus; that's its atomic number. And two different isotopes of a given element have the same number of protons (of course), but each has a different number of neutrons in its nucleus. Two different forms of the same element that have different numbers of neutrons in the nucleus. Each element has a number of different isotopes, and this is determined by the number of neutrons in the nucleus.

Isotopes of hydrogen are Protium, Deuterium and tritium. All of these three have same number of protons means same atomic number while these three differs in the number of neutrons means these have different atomic mass.

So mass of all isotopes are different while atomic number is same for all. So, protium's mass is 1u, mass of deuterium is 2u and mass of tritium is 3u.