Covalent  Bonds Page 29

In some compounds, atoms share their electrons. This is how water forms. An oxygen atom has space for two more electrons in its outer shell. A hydrogen atom has room for just one. Therefore the outer shells of two hydrogen atom can connect to the outer shell of one oxygen atom. Then they share a full set of electrons. This kind of bonding which is called  covalent. In addition, Covalent bonding is normally between two nonmetals like hydrogen and oxygen which have just a few spaces to fill in their outer electron shells.