Chemical bonds form when two atoms exchange or share one or more electrons. These bonds hold the atoms together to form compounds. There are two general types of bonds- ionic and covalent bonds. Covalent bonds form when two atoms share a pair of electrons. Neither atom completely loses or gains the electrons. The electron "orbit" both nuclei to some extent, but the electrons spend most of the time in the space between the bonded nuclei. There are two types of covalent bond. The first one is a nonpolar covalent bond, which the molecules are symmetrical and have equal electron sharing. Nonpolar covalent bond repel from one another, and they have lower melting and boiling points. They are often smelly (aeromatic) and slippery, and don't dissolve in water (hydrophobic). However, polar covalent bond has opposite characteristics. Polar covalent bonds have unequal electron sharing. In polar covalent bond, molecules are lop-sided creating dipoles or +/-. The attractive forces (van der waals forces) between molecules stabilize, and raise boiling and melting point. It dissolves in water (hydrophilic) and there are very high attractive forces called hydrogen bonding.
             Ionic bond is another type of chemical bonds. Ionic bonds form when electrons are completely removed from one atom and are completely gained by the second atom. The electrons are exchanged, and this is accompanied by the formation of two ions of opposite charge (cations and anions). The opposite charges will attract each other, and this attraction is what forms the chemical bond. These disassociate into ions as liquids or in solution. Some examples are, electric eels/rays, and nervous systems.
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