COVALENT BOND
Covalent bond is a chemical bond, which appeares as the result of formation of electronic pairs, common for the interacting atoms. Covalent bonds can be either polar or non-polar.
Non-polar covalent bond is a covalent chemical bond, which is formed between atoms with equal electro-negativity at the expense of their common electronic pairs. In molecules with this type of bond centers of positive and negative charges coincide.
Polar bond is a covalent chemical bond, which is formed by atoms, whose electro-negativities slightly differ. Centers of positive and negative charges in molecules with polar covalent bonds do not coincide. The electron cloud is shifted toward the atom with larger electro-negativity. Molecules of this kind are called dipoles.