Chemistry - A Molecular Science

1. Determine the number of

electrons

required (ER) to give each isolated non-hydrogen atom eight

electrons and each hydrogen two electrons. ER is eight times the number of non-hydrogen atoms (N

) plus two times the number of hydrogen atoms (NA

). H

ER = 8N

+ 2NA

(^) H

2. Determine the number of

valence

electrons in the molecule (VE), which is the number of

electrons that must be shown in the final structure.

3. The difference, ER - VE, divided by 2 is the number of

shared

pairs (SP).

SP = ½(ER - VE)

4. Draw the molecule with the bonded atoms c

onnected by single bonds. Then satisfy the number

of shared pairs required (SP) by adding double or triple bonds as necessary. Finally, add lone pairs of electrons to complete each atom’s oc

tet, but remember that hydrogen atoms have only

two electrons.

The following should also be considered when constructing a Lewis structure: •

The first atom in the formula is the least

electronegative and is generally the central atom.

(^) •
Hydrogen atoms have only one bond.
OCO
OCO
AB
Figure 5.8 Two possible Lewis structures of CO
(^2)

• The halogens are the elements of Group 7A (F, Cl, Br, and I).
  Multiple bonds should not be drawn to terminal halogen atoms.* More than one single bond •
  can be drawn to a halogen when it is the cent
  ral atom, as in the perchlorate ion (ClO
  1- 4
  ).
  Example 5.4
  Draw the Lewis structure for carbon dioxide, CO
  (^2)

1. ER = 3 atoms

x8 electrons/atom = 24 electrons.

2. VE = 4 from carbon + 6 from each oxygen = 4 + (2)(6) = 16 valence

electrons. Our Lewis structure must show eight pairs of electrons.

3. SP = ½

(24 - 16) = 4 pairs must be shared.

4. The four shared pairs can be distri

buted as two double bonds (Figure 5.8A)

or as a single and a triple bond (Fi

gures 5.8B). In Section 5.8, we explain

why structure A better represents the bonding in CO

. 2

5.7

RESONANCE

Resonance structures

are Lewis structures that differ onl

y in the placement of electrons.

The two structures of CO

shown in Figure 5.8 are two resonance structures. The most 2

common form of resonance results when multip

le bonds can be placed in more than one

position. When the resonance structures are clea

rly different, it is usually the case that only

one of the structures is important in the description of the bonding. Thus, only one of the two resonance structures shown for CO

in Figure 5.8 is expected to be important. 2

Chapter 5 The Covalent Bond

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