The Atom is the smallest unit of a elementary substance, i.e. of a substance which is not a compound of two or more elementary substances. All atoms consist of an atomic nucleus and an atomic shell. The nucleus is composed from several protons and neutrons, except the Atom of Hydrogen, where the nucleus is only one single proton. The shell consists of several electrons and the number of electrons in the shell is allways identical to the number of protons in the nucleus. Protons carry positiv electric charge, electrons negative charge and the absolute value of both charges is identical. Therefore in addition of all charges the atom in total carries charge 0.

If the number of electrons is not identical with the number of protons, the particle is no Atom but an Ion which can carry positive (cation) or negative (anion) charge.

The number of protons in the nucleus is very often identical to the number of neutrons but there are several exceptions.

Lithium Atom
Shematic view of an lithium atom. It is number 3 of the lightest Atoms existing.