Atomic Weight/Hydrogen Isotopes:

The symbols, atomic weights, and abundance of the three well-known isotopes of hydrogen are given in Table 2. ⁴₁H and ⁵₁H are also known.  Therefore, since they decay in fractions of a single second, that is not extensively studied.   Unless or else specified in this chapter, the word hydrogen involves deuterium, protium, and tritium. 1 will be used to refer to protium; confusion along with elemental hydrogen will be eliminated through spelling out the latter.