How do the number of valence electrons relate to an element's chemical properties?
Chemistry is conceived to be the result of the sharing and transfer of electrons from one atom to another.
Metals are electron rich materials, and are typically reducing agents. On the other hand, non-metals, to the right of the Periodic Table, have high nuclear charges, and accept electron density. It is not a coincidence that oxygen and fluorine, the 2 most oxidizing elements, are situated to the far right of the Periodic Table.