Most elements have outer electrons that are loosely bonded to the atom. In solid or liquid form these outer electrons become "delocalised" and are free to "wander around" the crystal lattice. Delocalised electrons give an element the properties of good thermal and electrical conductivity and the ability to reflect a high proportion of incident light.
These elements undergo chemical reactions by losing their outer electrons and forming positive ions. Roughly four fifths of the elements are metals.