Molecules with an odd number of electrons disobey the octet rule.
LEARNING OBJECTIVE [ edit ]
Describe the deviation from the octet rule by free radicals
KEY POINTS [ edit ]
While the majority of compounds formed from atoms belowatomic number 20 follow the octet
rule, there are many examples of compounds that do not.
Having an odd number of electrons in a molecule guarantees that it does not follow the octet rule,
because the rule requires eight electrons (or two for hydrogen) around each atom.
The most commonly encountered stable species that exist with an odd number of electrons
are nitrogen oxides, such as nitric oxide (NO) and nitrogen dioxide (NO2), both of which are free
radicals and disobey the octet rule.
TERMS [ edit ]
free radical
Any molecule, ion, or atom with one or more unpaired electrons. They vary in reactivity and
stability from highly reactive, occurring as transient (short­lived) species, tometastable.
metastable
Of or pertaining to a physical or chemical state that is relatively long­lived, but may decay to a
lower energy state when perturbed.
octet rule
Atoms lose, gain, or share electrons in order to have a fullvalence shell of eight electrons.
Hydrogen is an exception because it can hold a maximum of two electrons in its valence level.
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Free Radicals
Some elements, most notably nitrogen, can form compoundsthat do not obey the octet rule.
One class of such compounds are those that have an odd number of electrons. As the octet
rule requires eight electrons around each atom, a moleculewith an odd number of electrons
must disobey the octet rule. Molecules with unpaired electrons are termed 'free radicals.'
While typically highly unstable, and therefore highly reactive, some free radicals exhibit
stability of days, months, or even years. These latter compounds are said to be 'metastable,'
meaning they will decompose or react if given enough time, but are stable enough for a
considerable amount of time, from days to even years, when subjected to only minor
disturbances.
Examples of Free Radical Molecules
Recall that the Lewis structure of a molecule must depict the total number
of valence electrons from all the atoms which are bonded together.
Nitric oxide has the formula NO. The total number of valence electrons is 5+6=11. Therefore,
no matter how electrons are shared between the nitrogen and oxygen atoms, there is no way
for nitrogen to have an octet. It will have seven electrons, assuming that the oxygen
atom does satisfy the octet.
Nitric oxide
Nitric oxide (NO) is an example of a stable free radical. It does not obey the octet rule on the nitrogen
atom. Each line around the atoms represents a pair of electrons.
Nitric oxide is a by­product of combustion reactions that occur in engines, like those in
automobile engines and fossil fuel power plants. It is also produced naturally during the
electrical discharge of lightning during thunderstorms.
Nitrogen dioxide is the chemical compound with the formula NO2. Again, nitrogen dioxide
does not follow the octet rule for one of its atoms, namely nitrogen. The total number
ofvalence electrons is 5+2(6)=17. There is persistent radical character on nitrogen because it
has an unpaired electron. The two oxygen atoms in this molecule follow the octet rule.
Nitrogen dioxide
Nitrogen dioxide is another stable molecule that disobeys the octet rule. Note the seven electrons around
nitrogen. Formal charges and the molecule's resonance structures are indicated.
Nitrogen dioxide is an intermediate in the industrial synthesis of nitric acid, millions of tons
of which is produced each year. This reddish­brown toxic gas has a characteristic sharp,
biting odor and is a prominent air pollutant.