A neutral atom contains an equal number of positively charged particles, #"protons"#, massive nuclear particles, which are located in the central nucleus of the atom, and negative particles, #"electrons"#, fundamental negatively charged particles of negligible mass that are conceived to whizz about the nuclear core. The number of #"protons"# defines the identity of the atom, and this number #-=Z, "the so-called atomic number"#; the electronic structure defines its chemistry.
Atomic structure is completed by #"neutrons"#, massive nuclear particles of ZERO charge. Short range interaction between the #"protons"# and #"neutrons"# at nuclear distances is sufficient to overcome the electrostatic forces of repulsion.
Of course, ions can form, in which an atom has an excess or deficiency of electrons to give negative or positive ions. Elemental sodium has 11 protons in its nucleus; i.e. #Z=11#; how many electrons has it got; how many electrons does the sodium ion, #Na^+#, got?
