Question

Give a reason for the following:

The mass number of an atom is slightly less than the actual atomic mass.

Answer 1

Atomic mass = number of protons + number of neutrons + number electrons, and mass number = protons + neutrons
∴ Atomic mass = mass number + mass of electrons
Though the mass of electrons is very small 1/1837 of the hydrogen atom, yet the mass of electrons is there.
∴ Mass number is slightly less than the atomic mass.