Chemistry - A Molecular Science

Chapter 5 The Covalent Bond

5.4

LEWIS SYMBOLS OF THE ELEMENTS

The structure and bonding in a covalent molecule can be determined from its

Lewis

structures

, which is a representation of the molecu

le that shows the distribution of the

valence electrons of its constituent atoms. Thus, drawing Lewis structures is an important skill in chemistry, and one we will use frequently

. However, before we learn how to draw

Lewis structures of groups of bonded atoms, we must first consider the Lewis symbols of the atoms themselves. As shown in Figure 5.5, the Lewis symbol of an atom shows the valence electrons spread into four different

orbitals while obeying

Hund's rule. You should

note that this picture is not consistent with

the atomic electron configurations presented in

Chapter 2. For example, a carbon atom has a valence electron configuration of 2s

2 2p

2 , with

four valence electrons in three orbitals and only two unpaired electrons, while the Lewis symbol shows the four valence electrons as unpa

ired in four different orbitals. The reason

for this difference is that

Lewis symbols represent an atom that is about to bond, not an

isolated atom

, so the orbitals have already adopte

d the positions required for bonding. The

number of valence electrons for a main group

element is simply the element’s group

number, so the electron distributions given in the Lewis symbols are the same for all atoms in a group.

1A

2A Be Mg

3A B Al

4A

C Si

5A

N P

6A O

S

7A

8A He Ne Ar

H Li Na

F Cl

Figure 5.5 Lewis symbols for the elements of the first three periods

* There are exceptions to this rule when hydrogen atoms are ‘bridging’

atoms. For example, two BH

molecules are bridged by two 3

hydrogen atoms to form B

H 2

. The two bridging H atoms are 6

actually bonded to both boron atoms.

However, we will not consider

molecules of this type

further in this text.

In ionic compounds, nonmetals

gain

electrons from a metal

to attain filled valence

shells. In covalent compounds, nonmetals

share

electrons with other nonmetals

to attain

filled valence shells. A closed valence shell

for a nonmetal consists of eight electrons

(filled s and p sublevels), which is called an

octet

of valence electrons. Thus, nonmetals

strive to obtain an octet of valence electrons

when they bond. The tendency of nonmetals

to obtain eight valence electrons is known as the

octet rule

. In Chapter 4, we used the

octet rule to predict that the charge on an

anion was its Group Number - 8. We now use it

to determine the number of electrons that an

atom must share in its covalent bonds.

Hydrogen is an important exception to the octet rule

. This is because hydrogen’s

valence electrons are in the n = 1 shell, which can accommodate only two electrons. Thus, hydrogen requires only a duet of electr

ons in its covalent compounds,

which means that

only one bond is ever drawn to a hydrogen atom.*

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