Give a general introduction to the nucleus.

(N/A) In every atom,the positive charge and mass are densely concentrated at the center,which is called the nucleus of the atom.
The dimensions (radius) of a nucleus are much smaller than those of an atom.
Experiments on the scattering of $\alpha$-particles demonstrated that the radius of a nucleus is smaller than the radius of an atom by a factor of about $10^{4}$.
$\frac{\text{Volume of nucleus}}{\text{Volume of atom}} = \frac{4/3 \pi \times (10^{-15} \text{ m})^3}{4/3 \pi \times (10^{-10} \text{ m})^3} = 10^{-15} / 10^{-30} = 10^{-15}$ (approximate ratio based on standard nuclear radius $R = R_0 A^{1/3}$).
More simply,the volume ratio is approximately $10^{-12}$.
$\therefore$ 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 an atom enlarged to the size of a classroom,the nucleus would be the size of a pinhead.
Hence,the empty space in the atom is a very large area.