The Foundations of Chemistry

tronegative element is usually the one that needs the most electrons to fill its

octet. Example: CS 2 has the skeleton S C S.

b.Oxygen atoms do not bond to each other except in (1) O 2 and O 3 molecules; (2)

hydrogen peroxide, H 2 O 2 , and its derivatives, the peroxides, which contain the

O 22 group; and (3) the rare superoxides, which contain the O 2 group. Example:

The sulfate ion, SO 42 , has the skeleton

O

OSO

O

c. In ternary oxoacids, hydrogen usually bonds to an O atom, notto the central atom.

Example: nitrous acid, HNO 2 , has the skeleton H O N O. There are a few

exceptions to this guideline, such as H 3 PO 3 and H 3 PO 2.

d.For ions or molecules that have more than one central atom, the most symmet-

rical skeletons possible are used. Examples: C 2 H 4 and P 2 O 74 have the following

skeletons:

HH O O

C C and O P O P O

HH O O

2.Calculate N, the number of valence (outer) shell electronsneeded by all atoms in the
molecule or ion to achieve noble gas configurations. Examples:

A ternary oxoacid contains three

elements—H, O, and another

element, often a nonmetal.

7-5 The Octet Rule 283

For H 2 SO 4 ,

N 2    2 (H atoms) 1  8 (S atom) 4   8 (O atoms) 4  8  32  44 eneeded

For ClO 4 ,

N 8 (Cl atoms) 32 (O atoms) 40 eneeded

For NO 3 ,

N 1    8 (N atom) 3   8 (O atoms) 32 eneeded

Calculate A, the number of electrons availablein the valence (outer) shells of all the

atoms. For negatively charged ions, add to this total the number of electrons equal

to the charge on the anion; for positively charged ions, subtract the number of elec-

trons equal to the charge on the cation. Examples:

For H 2 SO 4 ,

A 2    1 (H atoms) 1  6 (S atom) 4   6 (O atoms) 2  6  24  32 eavailable

For ClO 4 ,

A 1    7 (Cl atom) 4  6 (O atoms) 1 (for 1charge) 7  24  1  32 eavailable

For NO 3 ,

A 1    5 (N atom) 3   6 (O atoms) 1 (for 1charge) 5  18  1  24 eavailable

Calculate S, total number of electrons sharedin the molecule or ion, using the rela-

tionship SNA. Examples:

^2 











^4 











For the representative elements, the

number of valence shell electrons in an

atom is equal to its periodic group

number. Exceptions: 1 for an H atom

and 2 for He.

For compounds containing only

representative elements, Nis equal to

8    number of atoms that are notH,

plus 2 number of H atoms.