Unstable elements tend to combine with other elements to form

Compounds

Explanation:

The reason why elements react is to attain stability. A compound is thus formed when two or more elements of different kind combine. For instance, An atom of sodium has only one electron in its outer shell. An atom of chlorine has seven outer electrons (almost a full shell). If the lone sodium outer electron is transferred to the chlorine atom, both atoms would have full outer shells and so would achieve stability. The end product is sodium chloride which is a compound.