Atoms fill their energy levels by sharing electrons.When atoms share electrons covalent bonds are formed. Neither atom losses an electron or gains an electron. Covalent bonds make the compound more stable. Covalent bonds are not easily broken because of this sharing of electrons. Further, they do not break apart when placed in water. Hydrogen is an example of a substance that uses covalent bonds. When two hydrogen atoms share an electron a hydrogen molecule is formed.
Covalent substances contain covalent bonds. For example, H2, HCl and H2O contains covalent bonds. Covalent bonds are formed by sharing a pair of electrons, inorder to become stable. They are held together by strong electrostatic forces between the nucleus (which is positively charged) and the electron pair which is negatively charged. Covalent bonds are often showed by dot and cross diagrams.