When one atom gives up or loses an electron, the atom will reduce in size. When an atom gains or receives an electron, the atom will grow in size. Ionic bonds lead to the formation of crystalline solids with high melting points and can be dissolved in water and other polar solvents. The ionic compounds formed through bonding are always solid and are good conductors of electricity. Ionic bonds are formed when a metallic element on the Periodic Table donates one or more electrons and forms a positive ion, or cation, and has a stable electron configuration. These positive ions enter a non-metallic element causing it to form a negative electron, or anion that has a stable electron configuration. Electrostatic forces attract cations and anions to each other. Ionic compounds include NaCl, or sodium chloride commonly known as table salt, and MgO, also known as magnesium oxide.

Covalent bonds do not give up electrons, but rather share electrons equally. Covalent bonding occurs because the atoms in a compound have a similar ability to gain and lose ions. Unlike ionic bonds that form between a metal and a non-metal, covalent bonds form between non-metals. Covalent...

Because atoms do not have to gain or lose electrons, multiple covalent bonds can form due to the atomic ability to share electron pairs, thus being able to form multiple bonds at the same time.
Covalency is considered strongest between atoms of similar electronegativities. Electronegativity is the tendency of an atom or a functional group to attract electrons to itself to form negative ions. Unlike ionic bonds, covalent bonding does not require that bonding atoms to be the same element, only that they share a comparable electronegativity. Covalent bonds are formed because it is easier to share electrons rather than gain or lose the number necessary for the atom to become a gas. Examples of covalent bonds are water that is composed of two hydrogen atoms and an oxygen atom (H20) and hydrochloric acid, HCl.

Reference List:

Decelles, P. 2001. "Chemical Bonds." The Entangled Bank, Dr. Paul's Virtually Biology Show.

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