Principles of Chemical Science_Lewis Structures Breakdown of the Octet Rule - Lec11

5.111 Lecture Summary #11 Wednesday, October 1, 2014

Readings for today: Section 2.9-2.12 (Same sections in 5th and 4th ed.)
Read for Lecture #12: Section 3.1 – The Basic VSEPR Model, Section 3.2 – Molecules with
Lone Pairs on the Central Atom. (Same sections in 5th and 4th ed.)
Topics:
Breakdown of the octet rule
Case 1. Odd number of valence electrons
Case 2. Octet deficient molecules
Case 3. Valence shell expansion

I. BREAKDOWN OF THE OCTET RULE
Case 1. Odd number of valence electrons
For molecules with an odd number of valence electrons, it is not possible for each atom
in the molecule to have an octet, since the octet rule works by pairing e-s.

Example: CH3
2) 3(1) + 4 = valence electrons
3) 3(2) + 8 = electrons needed for octet
4) 14 – 7 = bonding electrons
Radical species: molecule with an electron.

Most free radicals are very reactive. The reactivity of radical species leads to interesting
(and sometimes harmful) biological activity.

Free Radicals in Biology: a Paradox
)UHHUDGLFDOVSHFLHV '1$. Free radicals are essential for life.
Highly reactive oxygen radicals are a byproduct Free radicals are involved in critical
of metabolism and cause DNA damage. signaling pathways in or between cells,
and in critical enzymatic reactions.
Cigarette smoke also contains free radicals.




Figure by MIT OpenCourseWare.
Image from public domain.
O2 radicals are produced by our
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blood cells to kill invading
microorganisms.



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