According to Einstein mass energy equation i.e. E, where c is speed of light and Δm=[Zm_p+(A-Z)m_n]-M as we know when atom formed with nucleon and electrons there is energy needed to bind the nucleons in nucleus and this energy comes from above Einstein mass energy equation.

If B.E. represents the binding energy of H-atom which is numerically 13.6eV then,

equivalent mass of this energy [∵ B.E=Δmc²]

So mass of H-atom=m_p + m_e from this we can clearly conclude that mass of a H-atom is less than the sum of the masses of a proton and electron.