Chemistry - A Molecular Science

Chapter 5 The Covalent Bond

5.10

CHAPTER SUMMARY AND OBJECTIVES Covalent bonds result when bonding electrons are shared. The energy of the bound atoms is less than that of the separated atom

s because the bonding electrons experience the

positive charge of both nuclei. The energy required to break the bond is called the bond energy. When the electronegativities of the atoms are different, the bonding electrons are not shared equally to produce a bond dipo

le. The polarity of a bond increases as the

electronegativity difference between the two atoms increases. The less electronegative atom is written first in the formula and main

tains its name, while the more electronegative

element is written last, and

its ending is changed to -

ide

. The number of each atom present

in the formula is given by a prefix.

Lewis structures show the valence electrons

in a compound as dots if they are in lone

pairs or lines if they are bonding pairs. The number of shared pairs is determined as SP = 1 /^2

(VE - ER). The number of shared pairs in

a bond is called the bond order. Resonance

structures are Lewis structures that differ only

in the placement of the electron pairs.

Electron counting can be done with formal charge or oxidation state. Bonding

electrons are assigned equally to the bound at

oms in the formal charge, but they are

assigned to the more electronegative element in th

e oxidation state. Thus, formal charge is

a better description of charge distribution in purely covalent compounds, while oxidation state gives a better picture in very polar or ionic compounds.

After studying the material presented in this chapter, you should be able to:

1. describe the covalent bond (Section 5.1); 2. predict relative bond polarities (Section 5.2); 3. distinguish between an ionic and a covalent bond (Section 5.2); 4. name binary compounds formed from nonmetals (Section 5.3); 5. identify bonded and nonbonded electrons (Section 5.5); 6. determine the number of valence electrons

in a molecule (Sections 5.4 and 5.5);

7. draw Lewis structures (Section 5.6); 8. determine bond orders of all bonds in

a Lewis structure (Sections 5.5 and 5.6);

9. identify and draw different resonance forms (Section 5.7); 10.

distinguish between a formal charge

and an oxidation state (Section 5.8);

11.

assign formal charges and oxidation states

to the atoms in compounds (Section 5.8);

and

12.

identify important resonance forms based on formal charges (Section 5.8).

© by

North

Carolina

State

University