It separates isotopes according to charge and mass. Solids melt and liquids evaporate when the van der waals forces between molecules are broken. It contains two or more atoms. The protons are deep down in the nucleus and stay there. So electrons are either added or subtracted to make ions.
Free elements have zero as an oxidation number.
Only two electrons may occupy an orbital, but they must have opposite spins. Each column is a family of elements having similar properties. The properties are a periodic function of the atomic numbers. The table is based on the electron configuration of the atoms. The first column is the Alkali metals, they react vigorously with water to give hydrogen gas.