# (a) Distinguish between isotopes and isobars, giving one example for each.(b) Why is the mass of a nucleus always less than the sum of the masses of its constituents? Write one example to justify your answer.

(a)Isotopes are the atoms having the same atomic number

E .g.  $\inline _{17}^{35} \textrm{Cl}\; and\; _{17}^{37}\textrm{Cl}$

Isobars are the atoms  having the same mass number

Eg. $\inline _{2}^{3} \textrm{He}\; and\; _{1}^{3}\textrm{H}$

(b) Mass of a nucleus is less than its constituents because, in the bound state, some mass is converted into binding energy.

eg. mass of $\inline _{8}^{16} \textrm{O}\;$ nucleus is less than the sum of masses of 8 protons and 8 neutrons.

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