Wednesday, December 21, 2011

Over-rated Octet Rule

For decades students have been taught the Octet Rule, which states that atoms want to have eight electrons in their valence shells. When you know how many electrons an atom has in its outer shell, you know how it will gain, lose, or share electrons to form complete octets, which tells you the number of bonds the atom can form.  The real world of molecular bonding doesn't neatly conform to this rule, however. Hypervalent molecules form more than the expected number of bonds, appearing to stuff their valence shells with more than eight electrons. The greenhouse gas sulfur hexafluoride (SF6) is one example; another is chlorine trifluoride (ClF3), a gas used in rocket fuel, nuclear fuel processing, and etching operations in the semiconductor industry. It turns out, many molecules break the Octet "Rule."

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