Covalent compound?Properties of covalent compound?

A covalent compound is a compound in which the atoms that are bonded share electrons rather than transfer electrons from one to the other.

The reason that nonmetals have to share electrons with each other has to do with electronegativity.  Recall that electronegativity is a measure of how much an element pulls electrons away from other elements it is bonded to.  Metals generally have very low electronegativities (they don't much want to grab electrons) while nonmetals have high electronegativities (they really want to grab electrons).  The reason for this trend is the octet rule, which says that all elements want to have the same number of electrons as the nearest noble gas, because noble gases are unusually stable.  When metals bond to nonmetals, ionic compounds are formed because the metal atoms don't want electrons and easily give them to nonmetals that do want electrons.