Q.Define atomic mass unit ($u$).

Nuclear and atomic masses, when expressed directly in kilograms, come out as extremely inconvenient numbers (of order $10^{-25}$ kg or smaller), so nuclear physics instead uses a purpose-built unit called the atomic mass unit, symbol u. One atomic mass unit is defined as exactly one-twelfth of the mass of a single atom of the most abundant naturally occurring isotope of carbon, $^{12}_{6}C$:

Using this unit, key particle and atomic masses become simple, easily remembered numbers close to whole integers: the neutron's mass is 1.008665 u, the proton's mass is 1.007276 u, the hydrogen atom's mass is 1.007825 u, and by definition the mass of $^{12}_{6}C$ is exactly 12 u.

It is important to keep in mind that a mass quoted "for" a nucleus (or, more usually, "for" an atom) in atomic mass units is nearly always the atomic mass - the nucleus plus its full complement of orbiting electrons - rather than the bare nuclear mass alone; to isolate the true nuclear mass, the combined mass of the atom's $Z$ electrons must be subtracted from the tabulated atomic mass. Atomic masses are measured experimentally with an instrument called the Bainbridge mass spectrometer. …