Deuterium, an isotope of hydrogen, is represented symbolically as \({ }_{1}^{2} H\). This shows it contains one electron, one proton and one neutron. How can we arrive at this? \(" 1\) " inscribed on the lower portion of the symbol is the atomic number. The atomic number is the number of protons in the nucleus of the atom. Hence, there is only one proton for deuterium. For a neutral atom, the number of electrons equals the number of protons. Thus, there is also one electron in the shell of deuterium.
The mass number \(-2\) in this case - is the sum of protons and neutrons i.e. \(P + N =\) mass no, \(1+ N =2, N =1\).