An atom has a tiny centre called the nucleus. The nucleus is made of protons and neutrons. Protons are positively charged and neutrons have no charge, so the nucleus as a whole is positively charged.
Electrons move around the outside of the nucleus. Each electron is negatively charged. They are held in the atom by the attraction between their negative charge and the positive nucleus.
The nucleus is very small compared with the whole atom. Its radius is much smaller than the radius of the atom, so most of an atom is empty space. Even so, almost all of the mass of the atom is in the nucleus, because protons and neutrons are far heavier than electrons.
