1. State the octet rule and describe its basis.
2. Describe covalent bonding in molecules by drawing their Lewis structures.

Octet Rule. In our study of the periodic table we saw that the valence electron configuration was related to the chemical and physical properties of an element. The noble gas elements are the least reactive. and therefore the most stable group of elements. The Lewis dot symbols of the noble gas elements show eight valence electrons corresponding to filled s and p subshells. G. N. Lewis reasoned that when atoms enter into chemical combination they become more stable. He proposed that atoms gain or lose electrons until they have the same number of valence electrons as noble gas atoms, that is, eight. The octet rule states that when forming bonds atoms of the representative elements tend to gain, lose, or, share electrons until they have eight electrons in the valence shell.

Molecules are held together by bonds resulting from the sharing of electrons between two atoms in a manner that is consistent with the octet rule. A simple covalent bond is formed when two atoms in a molecule share a pair of electrons.

The formation of a covalent bond in hydrogen chloride can be represented with Lewis structures:

where the dash represents a covalent bond, or a pair of electrons shared by both the H atom and the Cl atom. By sharing the electron pair, the Cl atom has eight valence shell electrons. The stability of this bond results from both atoms acquiring a noble gas configuration. Notice that hydrogen is an exception to the octet rule. Rather than achieving an octet, it needs only two electrons to achieve a filled outer energy level. The H atom becomes isoelectronic with helium. The electron pairs on the Cl atom that are not involved in bonding are called lone pairs, unshared pairs, and nonbonding electrons. The sharing of valence electrons in methane and carbon tetrachloride is shown in Figure 9.2. The circles represent the valence shells of the atoms. They help to point out that each atom achieves an octet of valence electrons by sharing one or more pairs of electrons.

Figure 9.2 Sharing of electron pairs in CH₄ and CCl₄.

In some cases two or three pairs of electrons are shared by two atoms in order to reach an octet. In these molecules, multiple bonds exist. A double bond is a covalent bond in which two pairs of electrons are shared between two atoms, as between C and O in formaldehyde.

In general, atoms joined by a double bond lie closer together than atoms joined by a single bond. The C O bond length is shorter than the C—O bond length.

Nitrogen molecules (N₂) contain a triple bond.

Lewis Structures. Lewis structures represent the covalent bonding and location of unshared electron pairs within molecules and polyatomic ions. The steps for writing Lewis structures are as follows.

For application of this procedure see Example 9.5 and 9.6.

Draw the Lewis structure for hydrazine, N₂H₄. How many unshared electron pairs (lone pairs) are there on each N atom?

          Correct!

Click a Hint button for help.

1. Arrange the atoms in a reasonable skeletal form. H atoms form only one bond and so must be located on the outside of the atom.
   

   H	N	N	H
   	H	H

2. Count the valence electrons. Each N atom has 5 valence electrons and each H atom has 1. There are 2(5) + 4(1) = 14 valence electrons.

3. Connect the atoms with single bonds:

   

   Normally we would add unshared pairs to complete all octets of surrounding atoms, but in this case the H atoms only need the two electrons shared in the bond to the N atom. Counting the number of electrons used; 5 pairs = 10 valence electrons. Now add unshared pairs to complete the octets of the N atoms.

   

4. Count the electrons: 7 pairs = 14 valence electrons. This is the same number as given in step 2.

Complete the following questions to check your understanding of the material. Select the check button to see if you answered correctly.

1. How many lone pairs are on the underlined atoms in the following compounds?
   1. PH₃
   2. SCl₂
   3. H₂CO
2. Write the Lewis structures for the following species.
   1. NH₇⁺
   2. NCl₃
   3. CF₂Cl₂