The answer is that compounds are formed when elements are joined and held together by strong forces called chemical bonds. Covalent bonds share electrons between atoms in order to fill their electron shells. In the compound, molecules are held together by the attraction between the nucleus and the shared electrons.
Elements naturally have an outer shell of electrons, called the valence shell. the electrons in this shell are called valence electrons. Most elements have an incomplete valence shell, which would be any number under 8, since a full shell of electrons needs 8. These elements will form bonds to either share or take/give electrons to another element in order for their outer shell to be full.
