Lewis Symbols and the Octet Rule

Basic Concepts of Chemical Bonding

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

depends upon the electronic structures of the atoms
nature of the chemical forces within the compounds

A broad classification of chemical forces:

Ionic bonds
Covalent bonds
Metallic bonds

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

typically involves a metal with a nonmetal

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

typically involves one nonmetallic element with another

Metallic bonds

found in solid metals (copper, iron, aluminum)
each metal bonded to several neighboring groups
bonding electrons free to move throughout the 3-dimensional structure

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):

convenient representation of valence electrons
allows you to keep track of valence electrons during bond formation

consists of the chemical symbol for the element plus a dot for each valence electron

Sulfur

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

Note:

The dots (representing electrons) are placed on the four sides of the atomic symbol (top, bottom, left, right)
Each side can accommodate up to 2 electrons
The number of valence electrons in the table below is the same as the column number of the element in the periodic table (for representative elements only)

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