Chemical Bonds

Examine the terminology and characteristics of ionic bonds, covalent bonds, and valence electrons with this interactive tutorial.

Atoms are bound in a molecule by a force called the chemical bond. The atom’s outer electrons, called valence electrons, are involved in this chemical bonding.

An ionic bond is formed between two atoms when one atom transfers one or more electrons to another atom. An example of an ionic bond occurs within the sodium chloride molecule, where one electron is transferred from the sodium atom to the chlorine atom.

A covalent bond forms when atoms share electrons. Unlike the ionic bond, the “extra” electrons are not transferred. In the water molecule, covalent bonds exist between the oxygen and hydrogen atoms as the hydrogen electrons are shared with the oxygen atom.

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.

Get More Help!

Click one of these links to get more help from another Cramlet in this same chapter:

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