Question

why does hydrogen atom has less mass then the combined mass of proton and electron that makes it up?​

Answer 1

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In fact, even a normal hydrogen atom has a mass lower than that of a proton and electron by the 12.6 eV (electron volt) binding energy between proton and electron. Therefore the mass of the deuteron nucleus is lower than the combined mass of a proton and a neutron by about 0.1%.

Answer 2

Answer:

The proton and electron of a hydrogen atom represent a bound system. By a bound system we mean that energy has to be supplied to liberate the two particles from each other, to make them free. ... Therefore, the mass of the bound system is necessarily smaller than the sum of the rest masses of the constituent particles.