EXCEPTIONS TO OCTET RULE
1. Odd number of electrons (rare)
e.g.
NO,
11
NO2,
17
ClO2
19 number
of valence e
. ..
:N O:
octet on O
unpaired electron on N
free radical
2. Incomplete octet (number of e < 8, rare)
E.g.
F
BF3
F
F
B
B
F
F
F
F is too electronegative!
BF3 reacts readily with electron pair
donors (like NH3).
EXCEPTIONS TO OCTET
3. Molecules with atoms having more than
8 electrons
fairly common
never occur with second period atoms
occur with atoms in third period and
below
Atoms in third period and below are:
- larger
- have available (empty) d-orbitals
Conditions:
- large central atom
- highly electronegative atoms attached
to it: eg. F, Cl, O
- attached atoms can’t be too large
Examples:
NF5
PF5
AsF5
AsF6–
does not exist
stable
exists
stable
PF5
PCl5
PBr5
PI5
stable
less stable
unstable
unknown