Octet Rule

This might be the most important rule in all of undergraduate chemistry! Let’s start from the beginning and go from there.  

Nobel Gas Configuration

The tendency for atoms to gain or lose electrons to reach their Nobel gas configuration is known as the Octet Rule

LCAO of Nobel Gases

Concept: LCAO model proves why Nobel gases are so stable.   

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The LCAO of He2

Science-y Version: Helium would have to fill an anti-bonding molecular orbital in order to bond to itself, which undoes the extra stability provided by the filled bonding orbital, so it has no reason to make a bond.

Basic Version: Nobel gases don’t like to bond to anything!  

Periodic Table Patterns

  • Only worry about the elements on the first 2 or 3 rows. Living systems don’t have a lot of heavy metals!
  • Periods = Rows, Groups = Columns.
  • Remember your Group 1A through 8A elements. We will practice memorizing the top atom of each group. 

Octet Rule Details

  • First-row elements (H, He, Li) will prefer to possess 2 octet electrons
  • Second-row elements (C, N, O, F) will prefer to possess 8 octet electrons
  • Atoms smaller than carbon will possess less than 8 electrons: (Be) = 4 and (B) = 6
  • Third-row elements may form expanded octets that can hold up to (P) = 10 and (S) = 12 electrons.

Octet electrons is the name we give to ALL electrons that surround an atom. These help the atom reach its Nobel gas configuration.

  • Bonds = 2 Octet Electrons
  • Lone Pairs = 2 Octet Electrons

It seems like a lot to memorize, so let’s just knock out some practice problems. 

Problem: PRACTICE: Analyze the following molecules. Indicate ALL atoms that are in violation of the octet rule.

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From now on let’s just agree that hydrogens with 1 bond, and carbons with 4 bonds follow the octet rule, instead of multiplying them out every time. 

Problem: PRACTICE: Analyze the following molecules. Indicate ALL atoms that are in violation of the octet rule.

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You’ll keep seeing the octet rule a bunch more in this chapter. Let’s move on.