Ncert Exercises & Solutions

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

Hint: Some of the mass is lost in the form of binding energy.

Step 1: Find the mass defect.

Since, the difference in mass of a nucleus and its constituents,

∆ M

, is called the mass defect and is given by

∆ M = [{Zm}_{p} + (A - Z) m_{n}] - M

Step 2: Find the mass of the H-atom.

Also, the binding energy is given by,

B = mass defect (∆ M) \times c^{2} .

Thus, the mass of a H-atom =

m_{p} + m_{e} - \frac{B}{c^{2}}, where B = 13.6 eV is the binding energy .