Atomic model (not to schale)

Atom_diagram-texted-full.jpg The atomic number or the number of protons is indicated by Z (from the German Zahl ), the number of neutrons by N.

Too few or too many neutrons make the nucleus unstable, the isotope is radioactive.

The number of electrons is equal to the number of protons in the nucleus. The atom is electrically neutral. The number of electrons determines the chemical properties of the atom.

Atoms which differ only in the number of neutrons are called isotopes.

On earth most elements are dominated by one stable isotope. Chlorine is an execption with 75% Cl-35 and 25% Cl-37.
Elements like polonium, uranium and plutonium have no stable isotopes at all.

Click in picture to continue.     View image