Basic Concepts of Chemical Bonding

Lewis Symbols and the Octet Rule


Why are some substances chemically bonded molecules and others are an association of ions?

A broad classification of chemical forces:

  1. Ionic bonds
  2. Covalent bonds
  3. Metallic bonds

Ionic bonds - electrostatic forces that exist between ions of opposite charge

Covalent bonds - results from the sharing of electrons between two atoms

Metallic bonds

Lets look at the preferred arrangements of electrons in atoms when they form chemical compounds

Lewis Symbols and the Octet Rule

Valence electrons reside in the outer shell and are the electrons which are going to be involved in chemical interactions and bonding (valence comes from the Latin valere, "to be strong").

Electron-dot symbols (Lewis symbols):

Sulfur

Electron configuration is [Ne]3s23p4, thus there are six valence electrons. Its Lewis symbol would therefore be:

Note:

Atoms often gain, lose, or share electrons to achieve the same number of electrons as the noble gas closest to them in the periodic table

Because all noble gasses (except He) have filled s and p valence orbitals (8 electrons), many atoms undergoing reactions also end up with 8 valence electrons. This observation has led to the Octet Rule:

Atoms tend to lose, gain, or share electrons until they are surrounded by 8 valence electrons

Note: there are many exceptions to the octet rule (He and H, for example), but it provides a useful model for understanding the basis of chemical bonding.


1996 Michael Blaber