Chemistry - A Molecular Science

Chapter 5 The Covalent Bond

5.1

THE COVALENT BOND

Table 5.1

Electronegativities of the Main Group elements.

H 2.2 Li Be B C N O F 1.0 1.6 2.0 2.6 3.0 3.4 4.0 Na Mg Al Si P S Cl 0.9 1.3 1.6 1.9 2.2 2.6 3.2 K Ca Ga Ge As Se Br 0.9 1.0 1.8 2.0 2.2 2.6 3.2 Rb Sr In Sn Sb Te I 0.8 1.0 1.8 2.0 2.1 2.1 2.7 Cs Ba Tl Pb Bi Po At 0.8 0.9 2.0 2.3 2.0 2.0 2.2

r

o

E

-436kJ/mol

0.74 A

r

r

r

r

r

1

2

3

4

0 5

Figure 5.1 Interaction of two H atoms Energy of interaction between two H atoms as a function of r, the distance

between the nuclei, which

are represented by the small dots * Å is the Angstrom. 1Å = 10

-10 m. The Angstrom is a common unit

for bond lengths because most bond lengths fall between 1 and 3 Å. The picometer (pm) and nanometer are the common SI units for bond lengths. 1 pm = 10

-12

m and 1 nm = 10

-9 m, and most bond

lengths lie between 100 and 300 pm or 0.1 to 0.3 nm.

In Chapters 2 and 3, we discussed the energy of interaction between a nucleus and its electrons, which reduces their potential energy and is responsible for the existence of atoms. However, valence electrons can reduce their energy ever farther by interacting with more than one nucleus. Consider Figure 5.1, which shows the energy of interaction between two hydrogen atoms as a function of their separation, r. •

At a separation of r

, the two hydrogen atoms are too far apart to interact; 1

i.e.

, they have zero

energy of interaction, and the electrons are in spherical 1s orbitals.

(^) •
At a distance of r
, each electron is attracted by the nucl 2
eus of the other atom as well as its
own. There is also repulsion between the nuclei,
but it is much less because the nuclei are still
relatively far apart. The greater attractive
force between the oppositely charged particles
lowers the energy of the system. The electron density begins to concentrate in the region between the nuclei, distorting the orbi
tals from their spherical shape.
At r•
, the atoms are weakly boun 3
d together, and the two orbi
tals overlap one another.
At r•
, the energy of the system has 4
reached a minimum as the electron/nuclear attraction just
balances the internuclear repulsion. The bonding el
ectrons are in an orbital that concentrates
the electron density in the region between the two nuclei. The negative charge of the two electrons holds the positively charged nu
clei together in an H-H covalent bond.
At r•
, the repulsion between the nuclei is the dominat 5
ing force, so the energy rises sharply.
The separation between the nuclei at the minimum energy is called the
bond length
.
The bond length of the H-H bond is 0.74 Å (74
pm)*, which is quite short, while the I-I
bond length, which is nearly 2.7 Å, is a very long bond. The difference in the two bond lengths is due to the difference in size of the bound atoms (Figure 3.3). Smaller nuclei can get closer to other atoms than can larger nuclei.
The amount by which the energy of the two atoms is reduced by forming the bond is
known as the
bond energy

. Most bond energies lie between 100 and 1000 kJ/mol, so the

H-H bond energy (436 kJ/mol) lies in the middl

e of this range. The I-I bond energy, which

is 151 kJ/mol, is a relatively weak bond.

5.2

BOND POLARITY

The electronegativity (

) of an atom (Table 5.1) measχ

ures its ability to attract the bonding

electrons. Atoms with high electronegativities

have unfilled, low-energy orbitals and

strong affinities for bonding electrons. In H

the bound atoms have identical 2

electronegativities, so the bonding electrons are shared equally. However, if the electronegativities of the bound atoms differ, the bonding electrons are not shared equally

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