Give a general introduction to the nucleus. 

In every atom the positive charge and mass are densely concentrated at the centre of the atom which is called the nucleus of the atom.

The dimensions (radius) of a nucleus are much smaller than those of an atom.

Experiments on scattering of $\alpha$-particles demonstrated that the radius of a nucleus was smaller than the radius of an atom by a factor of about $10^{4}$.

$\frac{\text { Volume of nucleus }}{\text { Volume of atom }}=\frac{4 / 3 \pi \times\left(10^{-14}\right)^{3}}{4 / 3 \pi \times\left(10^{-10}\right)^{3}}=10^{-12}$

$\therefore$ The volume of a nucleus is about $10^{-12}$ times the volume of the atom.

Even so the nucleus contains most (more than $99.9 \%$ ) of the mass of an atom. If we think of a atom is enlarged to the size of a classroom, the nucleus would be the size of pinhead.

Hence, the empty in the atom is a very large area.