Octet Rule

If we accept the idea that nonmetals like to gain electrons, a logical next question is, "How many?" The simple answer is, "However many more will fit in that energy level." Because of the way that electrons fill up energy levels, a slightly better answer is "however many more electrons are needed to fill the s and p sublevels of that energy level." Usually that means "enough to make a total of eight in the outer shell." This principle is called the octet rule.

The octet rule says that atoms tend to gain, lose or share electrons so as to have eight electrons in their outer electron shell. It is a very useful rule but you should also know that there are many bonding situations where it does not apply. As you learn to use the octet rule, also learn to recognize situations where it does not apply and disregard it in those situations.

Practice

Take a moment to apply the octet rule to the elements for which you just drew Lewis diagrams (exercise 14) to determine how many electrons each of these atoms would like to gain. Keep in mind that hydrogen cannot possibly fit eight electrons into its outer shell. Check the answers that follow when you are done.

Answers

Hydrogen would like to gain one more electron. Notice that we start off with an exception to the octet rule because there is only room for two electrons in the first energy level. Carbon would like to gain four more electrons. Nitrogen has five valence electrons so it would like to gain three more. Oxygen has six valence electrons and thus would like to gain two more. Chlorine has seven valence electrons and would like to gain one more. Silicon has four valence electrons and would like to gain four more. Phosphorus has five valence electrons and would like to gain three more. Sulfur has six and would like to gain two more. Bromine has seven and would like to gain one more valence electron according to the octet rule.

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E-mail instructor: Eden Francis