The mass of a H-atom is less than the sum of the masses of a proton and electron. Why is this?

Explanation

Since, the difference in mass of a nucleus and its constituents, `DeltaM`, is called the mass defect and is given by
`DeltaM = [Zm_(p) + (A-Z)m_(n)] - M`
Also, the binding energy is given by `B =` mass defect `(DeltaM) xx c^(2)`
Thus, the mass of a H-atom is
`m_(p) + m_(e) - (B)/(c^(2))`, where ` B ~~ 13.6 eV` is the binding energy.