The atomic number indicates the number of protons within the core of an atom. The atomic number is an important concept of chemistry and quantum mechanics. An element and its place within the periodic table are derived from this concept.
When an atom is generally electrically neutral, the atomic number will equal the number of electrons in the atom, which can be found around the core. These electrons mainly determine the chemical behaviour of an atom. Atoms that carry electric charges are called ions. Ions either have a number of electrons larger (negatively charged) or smaller (positively charged) than the atomic number.