An atom's outermost electrons are called valance electrons. These valence electrons are the ones that are transferred (in the case of ionic bonds) or shared (in the case of covalent bonds).

Only the valence electrons of atoms can interact; the inner electrons do not form bonds.

A sodium atom, for example, has a total of 11 electrons -- two complete the first shell, eight complete the next shell, and a single electron occupies the outer shell. This single atom is a valence electron.

Atoms tend to gain or lose electrons until they have an outer shell that contains an octet (that is, eight electrons). This octet rule is based on the stability of noble gases, which have eight electrons in their outermost shells (except for helium) and thus do not form chemical bonds.

In our previous examples, in the sodium chloride molecule the chlorine atom picks up sodium's valence electron leaving eight in the outermost shells of both atoms. Similarly, the hydrogen electrons shared in the water molecule leave the oxygen molecule with eight electrons in its outermost shell.

There are three ways to represent chemical bonds in formula form.(See the tutorial above for visual representations of these formulas):

• Lewis Dot Formula - This model was developed by Gilbert Netwon Lewis in 1916, showing the bonding of the valence electrons in chemical bonds. Each atom is represented by its chemical symbol, with each symbol surrounded by dots that represent the valence electrons. This approach visually helps to reinforce the precepts of the octet rule.
• Structural Formula - This approach is a visual representation of the covalent bonds using a dash to represent the electron sharing between atoms. Again, chemical symbols are used for the atoms. Single, double, or triple lines between atoms represent single, double, or triple covalent bonds.
• Molecular Formula - This approach is the standard way of showing the combination of atoms to form molecules. Unlike the other approaches, it does not explicitly show how bonds are made.

Try one of these resources to learn more about chemical bonds:

• Chemical Bonds
• Molecular Resonance and Chemical Bonding 
• Polar and Non-Polar Bonds
• Simple Ionic Bond Builder