What is mass defect? Write down its formula.

(N/A) Mass defect is defined as the difference between the sum of the masses of individual nucleons (protons and neutrons) that constitute a nucleus and the actual observed mass of the nucleus.
It represents the mass that is converted into binding energy to hold the nucleus together.
The formula for mass defect $(\Delta m)$ is given by:
$\Delta m = [Z m_p + (A - Z) m_n] - M_{nucleus}$
Where:
$Z$ = Atomic number (number of protons)
$A$ = Mass number (total number of nucleons)
$m_p$ = Mass of a proton
$m_n$ = Mass of a neutron
$M_{nucleus}$ = Actual mass of the nucleus