Unit 2 Atoms and Bonding
2.88 Exceptions to the Octet Rule
Textbook ch 8.7
Big Idea 2: Chemical and physical properties of materials can be explained
by the structure and the arrangement of atoms, ions, or molecules and the
forces between them. Students will be able to demonstrate understanding
by laboratory investigation, analysis of data and creation of models.
Learning Objectives
:
SWBAT:
• Recognize exceptions to the octet rule and
draw accurate Lewis structures even when the
octet rule is not being obeyed
Exceptions to the Octet Rule
•
There are three types of ions or
molecules that do not follow the octet
rule:
1. Ions or molecules with an odd number of
electrons. (not covered on ap test
2. Ions or molecules with less than an octet.
3. Ions or molecules with more than eight
valence electrons (an expanded octet).
Odd Number of Electrons
Though relatively rare and usually quite
unstable and reactive, there are ions
and molecules with an odd number of
electrons.
This is not covered on AP test.
Comments About the Octet Rule
 2nd row elements C, N, O, F observe the octet rule
(HONC rule as well).
 2nd row elements B and Be often have fewer than 8
electrons around themselves - they are very reactive.
 3rd row and heavier elements CAN exceed the octet
rule using empty valence d orbitals.
 When writing Lewis structures, satisfy octets first,
then place electrons around elements having
available d orbitals.
Exceptions to the Octet Rule: Less than 8
Molecules
containing atoms of
Group 3A elements,
particularly boron
Six electrons around B
Four electrons around Be
Exceptions to the Octet Rule
More than 8
• If appropriate, more than eight electrons
can be put around elements from the
third or higher # periods. (Ex. 4,5,6,7)
• Atoms of third-period elements have 3d
orbitals and may expand their valence
shells to contain more than 8 electrons
More Than Eight Electrons
• The only way PCl5 can
exist is if phosphorus
has 10 electrons around
it.
• It is allowed to expand
the octet of atoms on
the 3rd row or below.
• Can exceed octet rule
using their empty
d orbitals
• Especially when
surrounded highly
electronegative atoms
like Cl, Br, O
Sulfur 10 electrons
Sulfur 12 electrons
Xe with 12 electrons
• Many other molecules contain central atoms
with more than an octet of electrons.
• Some Are:
• Phosphorus • Halogens (except for fluorine)
• Krypton
• Sulfur
• Xenon
• Selenium
References
Our textbook: Brown, Lemay et all. AP edition
chemistry, 13th edition, 2015
I modified the information to fit our needs in AP
Chemistry needs.