Question

n also how to calcute the mass of hydrogen ...(3 isotopes?)

Answer 1

Hydrogen (1H) has three naturally occurring isotopes, sometimes denoted 1H, 2H, and 3H. The first two of these are stable while 3H has a half-life of 12.32 years. All heavier isotopes are synthetic and have a half-life less than one zeptosecond (10−21 second). Of these, 5H is the most stable, and 7H is the least.
this is term of chemistry.