In chemistry and physics, the atomic number of an atom is the number of protons in the atom. The chemical elements of the periodic table are listed in order of atomic number.
Atoms also have electrons and neutrons. The number of electrons and neutrons do not change the Atomic number of an element. The atomic weight does change as more neutrons are in the nucleus of the atom.
The number of electrons change the charge of the atom. If the number of electrons does not equal the number of protons, the atom is called an ion.