An atom has a tiny nucleus at its centre. The nucleus contains protons, which are positively charged, and neutrons, which have no charge, so the nucleus has an overall positive charge. The nucleus is surrounded by negatively charged electrons that move around it in shells. An atom is neutral overall because it has the same number of electrons as protons.
The nucleus is much smaller than the atom. Its radius is about one ten-thousandth of the radius of the whole atom. If an atom were enlarged until its radius was 100 m, the nucleus would have a radius of only about 1 cm. This means that most of the volume of an atom is empty space.
Although the nucleus is so small, it contains almost all of the mass of the atom. Protons and neutrons are much heavier than electrons, and the electrons have almost no mass.
