Elements that react to form positive ions are metals. Elements that do not form positive ions are non-metals. Most elements are metals. They are found to the left and towards the bottom of the periodic table, while non-metals are found towards the right and top.
Position links to atomic structure. A metal atom has only a few electrons in its outer shell, usually one, two or three. It reaches a full outer shell by losing these electrons, which makes a positive ion. A non-metal atom has more outer electrons, usually four to seven. It reaches a full outer shell by gaining electrons to make a negative ion, or by sharing electrons with other atoms. The reactions of an element depend on the arrangement of its electrons, and so on its atomic number.
The two types also differ in their physical properties. Metals are good conductors of heat and electricity, are malleable so they can be hammered into shape, are shiny and usually have high melting points. Solid non-metals are poor conductors, are brittle so they break when hit, are dull and usually have low melting and boiling points.
