The Foundations of Chemistry

Step 4a: Now we have satisfied the octet rule, but we have used only 32 of the 34 e

available. We place the other two electrons on the central S atom.

Calculation of the formal charge shows that

for S, FC 6 (42) 0

for F, FC 7 (16) 0

EXAMPLE 7-9 Limitations of the Octet Rule

Write the Lewis formula for the triiodide ion, I 3 .

Plan

We apply the usual stepwise procedure. The calculation of SNAin step 2 shows only
2 eshared, but a minimum of 4 eare required to bond two I atoms to the central I. Limi-
tation type D applies, and we proceed accordingly.

Solution

Step 1: The skeleton is [I I I]

Step 2: N 3    8 (for I) 24 eneeded

Step 3: A 3    7 (for I) 1 (for the 1charge) 22 eavailable

Step 3: SNA 2 eshared. Two I atoms are bonded to the central I. This

requires a minimum of 4 e, but only 2 ehave been calculated in step 4.

This is therefore an example of limitation type D.

Step 2a: Increase Sfrom 2 eto 4 e.

Step 3:

Step 4:

Step 4a: Now we have satisfied the octets of all atoms using only 20 of the 22 eavail-

able. We place the other two electrons on the central I atom.

Calculation of the formal charge shows that

for I on ends, FC 7 (16) 0

for I in middle, FC 7 (26) 1

You should now work Exercise 58.

We have seen that atoms attached to the central atom nearly always attain noble gas config-

urations,even when the central atom does not.



I I I or



III

[ I I I ]

[I I I]

F

F F

F

S

F

F F

F

S In SF^4 , sulfur has an expanded valence

shell.

7-7 Limitations of the Octet Rule for Lewis Formula 295

The central iodine atom in I 3 has an

expanded valence shell.