The Foundations of Chemistry

Let us see how this relationship applies to some species whose Lewis formulas we have

already shown.

For F 2 ,

N 2    8 (for two F atoms) 16 eneeded

A 2    7 (for two F atoms) 14 eavailable

SNA 16  14  2 eshared

The Lewis formula for F 2 shows 14 valence electrons total, with 2 eshared in a single

bond.

For HF,

N 1    2 (for one H atom) 1   8 (for one F atom) 10 eneeded

A 1    1 (for one H atom) 1   7 (for one F atom) 8 eavailable

SNA 10  8  2 eshared

The Lewis formula for HF shows 8 valence electrons total, with 2 eshared in a single

bond.

For H 2 O,

N 2    2 (for two H atoms) 1  8 (for one O atom) 12 eneeded

A 2    1 (for two H atoms) 1  6 (for one O atom) 8 eavailable

SNA 12  8  4 eshared

The Lewis formula for H 2 O shows 8 valence electrons total, with a total of 4 eshared,

2 ein each single bond.

For CO 2 ,

N 1    8 (for one C atom) 2   8 (for two O atoms) 24 eneeded

A 1    4 (for one C atom) 2   6 (for two O atoms) 16 eavailable

SNA 24  16  8 eshared

The Lewis formula for CO 2 shows 16 valence electrons total, with a total of 8 eshared,

4 ein each double bond.

For NH 4 ,

N 1    8 (for one N atom) 4   2 (for four H atoms) 16 eneeded

A 1    5 (for one N atom) 4   1 (for four H atoms) 1 (for 1charge) 8 eavailable

SNA 16  8  8 eshared

The Lewis formula for NH 4 shows 8 valence electrons total, with all 8 eshared, 2 e

in each single bond.

The following general steps describe the use of the SNA relationship in

constructing dot formulas for molecules and polyatomic ions.

Writing Lewis Formulas

1.Select a reasonable (symmetrical) “skeleton” for the molecule or polyatomic ion.

a.The least electronegative elementis usually the central element, except that H is

never the central element, because it forms only one bond. The least elec-

282 CHAPTER 7: Chemical Bonding

F F

H F

H O

H

O C O

The 1ionic charge is due to a
deficiencyof one erelative to the
neutral atoms.

H

H

H N H