Question

Explain why ${}_1{}^3\mathrm{H}$ and ${}_2{}^3\mathrm{H}$ are not considered isotopes.

Answer 1

Isotopes are the atoms of an element with the same atomic number but different mass numbers. There are three isotopes of hydrogen atom, namely protium $\left({}_1{}^1\mathrm{H}\right)$, deuterium $\left({}_1{}^2\mathrm{H}\right)$ and tritium $\left({}_1{}^3\mathrm{H}\right)$. ${}_1{}^3\mathrm{H}$ and ${}_2{}^3\mathrm{H}$ are not considered as isotopes because they do not have the same atomic number. Such atoms that have same mass number but different atomic numbers are called isobars.