When atoms exchange or share electrons they form chemical bonds . The particles which are formed by bonded atoms sharing electrons are called molecules . Chemical reactions occur when new bonds are formed or broken in molecules. Covalent bonds are formed when two atoms share pairs of electrons. The particular pattern and geometry of bonding defines the basic structure of molecules.
Covalent bonds are generally strong and very stable at physiological temperatures.
Sometimes sharing a single pair of electrons is not enough to fill the outer shells of atoms and create a stable molecule. In that case, atoms may share two or more pairs of electrons, forming double or even triple bonds . If the electrons in a covalent bond are shared equally between the atoms, the bond is nonpolar . Polar bonds are formed when an atom tends to attract the shared electrons more than its bonded partner does, resulting in partial negative and partial positive charge on the two atoms, respectively. This attraction forms a weak bond called a hydrogen bond .