5.5
LEWIS STRUCTURES OF DIATOMIC MOLECULES
Cl
Cl
+Cl
Cl
bonding pairlone pairsFigure 5.6 Lewis structure of Cl(^2)
The two Cl atom in Cl
must share two electrons (one pair) to 2
achieve octets. One electron from each Cl is used to form the bonding pair. The bonding pair is highlighted by the blue area, while the red and yellow circles highlight the valence electrons on each atom. The colored regions are shown for emphasis only and are not used in subsequent Lewis structures. H+ H
HHorHH
®
Figures 5.7a Lewis structure of H
2
H^2
contains one shared pair but no lone pairs. O+ O
OOorOO
®
Figure 5.7b Lewis structure of O
(^2)
O^2
contains two shared pairs and four lone pairs.
N+ N
NNorN
®
N
Figure 5.7c Lewis structure of N
(^2)
N^2
contains three shared pairs and two lone pairs.
Lewis structures of molecules are obtained by giving each atom (except hydrogen) an octet of valence electrons. Consider the covalent bond in Cl
, shown in Figure 5.6. The 2
separated chlorine atoms each have 7 valen
ce electrons, but they each achieve an octet by
sharing their unpaired electrons. The bonding pair is shared equally by the two atoms because their electronegativities are identical; consequently the bond is nonpolar.
Each chlorine atom in Figure 5.6 has three pairs of electrons that are not involved in
bonding and are called
nonbonding electrons
or
lone pairs
. Thus, the Cl
molecule 2
contains six lone pairs (three pairs on
each chlorine atom) and one bonding pair.
The number of electrons that must be shared in a molecule is the difference between
the number ofelectrons
required (
ER) to give each atom an octet (or duet for H) with no
sharing and the number of electrons availabl
e in the molecule, which is the number of
valence
electrons (
VE). The number of
shared
pairs (
SP) is one-half the number of shared
electrons, so we write:
SP=
[½
ERVE].
For example, the number of electrons
required without sharing in Cl
is: 2
ER= (2 atoms)(8 electrons needed per atom) = 16, and
the number of valence electrons available is:
VE= (2 atoms)(7 valence electrons each) =
- The number of shared pairs of electrons in Cl
is: 2
SP=
½[16 - 14] = 1.
Now, let’s consider the Lewis structures of H
, O 2
and N 2
. 2
For H(Figure 5.7a), 2
ER= (2 atoms)(2 e1- needed/atom) = 4 e1- requiredVE= (2 atoms)(1 e1-/atom) = 2 valence e1- availableSP=½
(4 - 2) = 1 pair must be sharedFor O(Figure 5.7b), 2
ER= (2 atoms)(8 e1- needed/atom) = 16 e1- requiredVE= (2 atoms)(6 e1-/atom) = 12 valence e1- availableSP=
½(16 - 12) = 2 pairs must be sharedFor N(Figure 5.7c), 2
ER= (2 atoms)(8 e1- needed/atom = 16 e1- requiredVE= (2 atoms)(5 e1-/atom) = 10 valence e1- availableSP=½
(16 - 10) = 3 pairs must be sharedAn electron pair can be represented as two dot
s or one line. In this text, we use the
more common convention of representing bonding pairs as lines and lone pairs as dots. Thus,
each line in a bond represents two electrons
. Note that the number of electrons
Chapter 5 The Covalent Bond© byNorthCarolinaStateUniversity