Z,A and M represent the atomic number, mass number and rest mass of a nucleus.
a. Show that 'M' is always less than the mass of the constituent particles.
b. What is this mass difference called ?
c. Give the relation.

Explanation

a. Protons and neutrons come together within a very small space of the order of `10^(-14)m` to form a nucleus. The energy required to do so is provided by nucleus at the expense to their masses. Hence the reason.
e.g. For `""_5B^(10)`, the mass , `M = 10.012944`
`""_(5)B^(10)` consists of 5 protons and 5 neutrons.
Hence total mass = `m_P + m_N`
`= 5 xx "Proton mass " + 5 xx "Neutron mass"`
`= 5 xx 1.007825 + 5 xx 1.008665`
`= 5.039125 + 5.043325 = 10.08245 u`.
i.e., `M < (m_P + m_N)`, hence the reason.
b. This mass difference is called mass defect (`Delta m)`
c. `Deltam = Zm_P + (A - Z) m_N - M`.