Atoms are considered to be the building blocks of "ordinary" matter. A single atom is made up of a central nucleus with electrons around the outside.
So what's in the nucleus?
There are two types of particle in the nucleus of an atom: the proton and the neutron. The proton has a positive charge. The number of protons in a nucleus is the same for all the atoms of a particular element. It corresponds to the atomic number of that element. For example, if a nucleus contains three protons, then it is part of an atom of lithium (atomic number 3).
The neutron has a similar mass to the proton but has no charge. The neutrons help keep the nucleus together.