If there are too many electrons it is a negative ion (since electrons have a negative charge.) If it has too few electrons it is a positive ion. If an atom has too many or too few electrons, it is called an ion. (Specifically one atomic mass unit equals 1/12th the mass of a carbon atom, but it is easiest just to look up the atomic mass on the Periodic Table.)Ītoms are normally neutral, that is there are the same number of protons (positive charges) in an atom, as there are electrons (negative charges). For example, copper has an atomic number of 29 (29 protons) but an atomic mass, not of 58 but of 63 meaning it has 34 neutrons. The exceptions are (1) Hydrogen, which is found most often as a proton with an electron and no neutron, and (2) the heaviest elements, which need more neutrons to glue the atom together keeping the protons from repelling each other. So the atomic mass of He is (2 protons + 2 neutrons) roughly 4 (plus a little more). Most light atoms have the same number of neutrons as protons. The atomic mass is roughly the number of protons + neutrons (plus a little more for the electrons and the fact that a neutron is slightly more massive than a proton). Hydrogen (H) has one proton in its nucleus, Helium (He) has 2, and so on.

Each has an atomic number, the number of protons in that atom. All chemical elements are listed in the Periodic Table of Elements. The chemical elements are the basic atoms, from which all chemical compounds are made.

H mass periodic table