The atomic weight is calculated by adding the number of protons and neutrons together. Atomic masses are not listed as whole numbers on the periodic table because atoms can come in forms with different amounts of neutrons. The atomic weight reported for any particular element is an average weight of all the known forms of that element. An electron is a negatively charged particle found circling or orbiting an atomic nucleus. An electron, like a proton is a charged particle, although opposite in sign, but unlike a proton, an electron has negligible atomic mass.
Electrons contribute no atomic mass units to the total atomic weight of an atom.
Neutral atoms have the same number of protons and electrons. Since the charge of these two particles are equal in magnitude and opposite in sign, the charge of a neutral atom is zero. When an atom gives up or takes on an electron, the positive and negative charges are no longer balanced. Extra electrons give an ion a negative charge. Fewer electrons give an ion a positive charge.