What is mass defect?

Hint: When an atom breaks up into its component particles, it is observed that the mass of the atom is less than the sum of all masses of the component particle. This difference between the masses is determined by the mass defect. The mass defect also accounts for the stability of an atom.

Complete answer:
When an atom breaks up into its components, it is observed that the mass of the whole atom as a whole is less than the sum of the mass of its components. This means that the mass of the parent atom is less than the mass of the sum of its daughter atoms.
When electrons and neutrons combine together to form an atom, energy is released due to this makes the atom stable. Therefore, the sum of mass of component particles is less than the mass of the atom.
The mass defect is the difference between the sum of mass of component particles and the mass of the atom. The greater the mass defect for an atom, the more stable is the atom.
Stability of the atom is determined by the binding energy of an atom, it is given by-
$E=\Delta m{{c}^{2}}$
$E$ is the atom’s binding energy
$\Delta m$ is the difference in mass or mass defect
$c$ is the speed of light
Therefore, the mass defect is the difference in the sum of mass of component particles of an atom to the mass of the atom. The greater the mass defect, the more stable an atom is.

Note:
The less energy an atom possesses the more stable it is. The mass defect follows the law of conservation of mass and energy. The binding energy of an atom is the energy required to separate a particle from the system of atoms. Binding energy per nucleon is calculated by dividing the total binding energy by the total number of nucleons.