The Foundations of Chemistry

see limitation type B

g

Step 2: N 3    8 (for Cl) 1   6 (for B) 30 eneeded

Step 3: A 3    7 (for Cl) 1   3 (for B) 24 eavailable

Step 3: SNA 30 e 24 e 6 eshared

Cl

Step 3: ClShBSCl

Step 4:

Each chlorine atom achieves the Ne configuration. The boron (central) atom acquires a share
of only six valence shell electrons. Calculation of formal charges shows that

for B, FC 3 (30)0 and for Cl, FC 7 (16) 0

You should now work Exercise 54.

EXAMPLE 7-7 Limitations of the Octet Rule

Write the Lewis formula for phosphorus pentafluoride, PF 5 , a covalent compound.

Plan

We apply the usual stepwise procedure to write the Lewis formula. In PF 5 , all five F atoms are
bonded to P. This requires the sharing of a minimum of 10 e, so this is an example of limi-
tation type D. We therefore add the extra step 2a, and increase Sfrom the calculated value of
8 eto 10 e.

Solution

Step 1: The skeleton is

Step 2: N 5    8 (for F) 1    8 (for P) 48 eneeded

Step 3: A 5    7 (for F) 1    5 (for P) 40 eavailable

Step 3: SNA 8 eshared

Step 2: Five F atoms are bonded to P. This requires the sharing of a minimum of

10 e. But only 8 ehave been calculated. This is therefore an example of

limitation type D.

Step 2a: Increase Sfrom 8 eto 10 e, the number required to bond five F atoms to

one P atom. The number of electrons available, 40, does not change.

Step 3:

F

F

F F

F

P

Cl BCl or

Cl Cl

Cl BCl

7-7 Limitations of the Octet Rule for Lewis Formulas 293

F

F

F F

F P