Chemistry - A Molecular Science

the type of bond, ionic or covalent, must fi

rst be established. If the bonds are assumed to

be covalent, the resulting charge on the atom is called its

formal charge

, but if they are

assumed to be ionic, the charge is the atom’s

oxidation state

. Thus, oxidation states better

describe ionic substances, while formal ch

arge is a better description in covalent

substances. However, as the bonds in a covalent

substance become more polar, the charge

distribution becomes more like the oxidation states. The

assigned charge

on an atom in a

molecule is equal to the number of valence electrons in the free (neutral) atom minus the number of valence electrons assigned to the at

om in the molecule. The number of valence

electrons in the free atom (

VE

) is given by the atom’s group number. The number of

electrons assigned to the atom in the molecule is the number of nonbonding electrons (

NB

)

shown in the Lewis structure plus those bonding electrons (

BE

) that are assigned to it. In

the following, we show that the only differe

nce between the formal charge and oxidation

state of an atom is how

BE

is determined.

FORMAL CHARGE To determine the

formal charge (

FC

on an atom, we assume that all of the bonds are )

purely covalent. In a covalent bond, the bonding electrons are shared equally, so each atom is assigned all of its nonbonding electrons (

NB

) but only

half

of its bonding electrons

( ½

BE

). As a result, the formal charge of atom A (

FC

is determined to be A)

FC

= VE - (NB + ½A

BE) Eq.

5.1

The sum of the formal charges must equal th

e charge on the species (zero for a molecule).

For example, the sum of the formal charg

es on the nitrogen and three oxygen atoms in

NO

1- 3

must equal -1, the charge on the ion.

N

HNH

H

Figure 5.10 Lewis formulas of a nitrogen atom and the ammonia molecule

Consider the nitrogen atom in ammonia shown in Figure 5.10. A free nitrogen atom

has five valence electrons (

VE = 5

), but the nitrogen atom in NH

is surrounded by six 3

bonding electrons and two nonbonding electrons

. The N-H bonding electrons are assumed

to be shared equally between N and H when

determining formal charge, so only one-half

of the bonding electrons are assigned to the nitr

ogen. The formal charge on the nitrogen is

determined with Equation 5.1 to be

FC

= 5 - [2 + N

1 /^2

(6)] = 0

. For the H atoms,

NB = 0

and

BE

= 2

, so

FC

= 1 - [0 + H

1 /^2

(2)] = 0

. We conclude that there is no non-zero formal charge in

ammonia, and the sum of the formal charges is zero, which it must be for a molecule.

Chapter 5 The Covalent Bond

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State

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