One of the fundamental ideas of modern chemistry is that all matter is made up of atoms. The atoms themselves are composed of smaller particles. For our purposes these particles include the following three:
Most of the mass of an atom comes from the first two which together coexist in the center of the atom called the nucleus. It is the number of protons which is called the atomic number (Z, from the German Zahl or "number") and differentiates one element from another. The atomic numbers are the sequential integers on the periodic table which currently number the elements from 1 to 116.
Neutral atoms (as opposed to ions) have a number of electrons equal to the number of protons in the nucleus. Electrons are found in the space surrounding the nucleus rather than inside it.