Ch. 9.9: Exceptions to the Octet Rule
o Incomplete Octet
§ Atoms with fewer than 4 valence electrons cannot achieve octet: BeH2, BF3:
F
H
F
H
Be
B
F
§
These are stable but react with electron-rich species:
H
F
F
+
B
N
F
H
B
F
H
F
H
N
F
H
H
§ Unbonded electrons on N attracted to electron-deficient B
o Odd Number of Valence Electrons
§ Some species can not complete pairs due to odd numbers: NO, NO2:
N
§
O
N
O
O
Odd-numbered species called radicals—are very reactive
§ NO2 monomers dimerize to form bonds between N atoms:
O
O
N
+
O
N
O
O
N
O
O
N
O
• Two lone electrons form single bond
o Expanded Octets
§ Atoms in 3rd energy level and higher (e.g. P, S, Cl, As, Se, Br, Kr, I, Xe) can accommodate more than 8
electrons (current evidence is that d orbitals are not involved): PCl5, SF6:
SF6 can be formed without counting ALL electrons by placing all 6
F
Cl
F atoms around S, then subtracting bonds from S valence electrons
F
F
Cl
Cl
to show that none are left. This only works with terminal atoms
S
P
that form only one bond, with no chance of double bonding. See
F
F
examples below:
Cl
Cl
F
§ Some expanded octets not as obvious: BrCl3, XeF4:
Place 3 Cl atoms around Br and
Br has 7 valence electrons, so it is
Cl
connect each with a single bond:
missing 4 electrons, which are
placed on it in pairs. Fill in the
Br
rest of the octets on Cl to give the
Cl
Cl
full 28 valence electrons.
For XeF4, connect the 4 F atoms
to Xe with a single bond:
F
F
Xe
F
F
Xe has 8 valence electrons, so it is
missing 4 electrons, which are
placed on it in pairs. F’s octets are
completed for 36 electrons
Cl
Br
Cl
Cl
F
F
Xe
F
F
Exceptions to Octet Rule Notes.docx
•
p.2
Formal Charge Considerations
§ Some species have an option of maintaining an octet or creating an expanded octet with less formal charge,
e.g. PO43–:
O
O
P
O
•
O
O
O
P
O
etc.
O
The structure that maintains the octet [on left] is preferred over the expanded octet, even though the
expanded octet reduces the formal charges.