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Resonance

You may have noticed that the nitrite anion in [link] can have two possible structures with the atoms in the same positions. The electrons involved in the N–O double bond, however, are in different positions:

Two Lewis structures are shown. The left structure shows an oxygen atom with three lone pairs of electrons single bonded to a nitrogen atom with one lone pair of electrons that is double bonded to an oxygen with two lone pairs of electrons. Brackets surround this structure, and there is a superscripted negative sign. The right structure shows an oxygen atom with two lone pairs of electrons double bonded to a nitrogen atom with one lone pair of electrons that is single bonded to an oxygen atom with three lone pairs of electrons. Brackets surround this structure, and there is a superscripted negative sign.

If nitrite ions do indeed contain a single and a double bond, we would expect for the two bond lengths to be different. A double bond between two atoms is shorter (and stronger) than a single bond between the same two atoms. Experiments show, however, that both N–O bonds in NO 2 have the same strength and length, and are identical in all other properties.

It is not possible to write a single Lewis structure for NO 2 in which nitrogen has an octet and both bonds are equivalent. Instead, we use the concept of resonance    : if two or more Lewis structures with the same arrangement of atoms can be written for a molecule or ion, the actual distribution of electrons is an average of that shown by the various Lewis structures. The actual distribution of electrons in each of the nitrogen-oxygen bonds in NO 2 is the average of a double bond and a single bond. We call the individual Lewis structures resonance forms    . The actual electronic structure of the molecule (the average of the resonance forms) is called a resonance hybrid    of the individual resonance forms. A double-headed arrow between Lewis structures indicates that they are resonance forms. Thus, the electronic structure of the NO 2 ion is shown as:

Two Lewis structures are shown with a double headed arrow drawn between them. The left structure shows an oxygen atom with two lone pairs of electrons double bonded to a nitrogen atom with one lone pair of electrons that is single bonded to an oxygen atom with three lone pairs of electrons. Brackets surround this structure, and there is a superscripted negative sign. The right structure shows an oxygen atom with three lone pairs of electrons single bonded to a nitrogen atom with one lone pair of electrons that is double bonded to an oxygen atom with two lone pairs of electrons. Brackets surround this structure, and there is a superscripted negative sign.

We should remember that a molecule described as a resonance hybrid never possesses an electronic structure described by either resonance form. It does not fluctuate between resonance forms; rather, the actual electronic structure is always the average of that shown by all resonance forms. George Wheland, one of the pioneers of resonance theory, used a historical analogy to describe the relationship between resonance forms and resonance hybrids. A medieval traveler, having never before seen a rhinoceros, described it as a hybrid of a dragon and a unicorn because it had many properties in common with both. Just as a rhinoceros is neither a dragon sometimes nor a unicorn at other times, a resonance hybrid is neither of its resonance forms at any given time. Like a rhinoceros, it is a real entity that experimental evidence has shown to exist. It has some characteristics in common with its resonance forms, but the resonance forms themselves are convenient, imaginary images (like the unicorn and the dragon).

The carbonate anion, CO 3 2− , provides a second example of resonance:

Three Lewis structures are shown with double headed arrows in between. Each structure is surrounded by brackets, and each has a superscripted two negative sign. The left structure depicts a carbon atom bonded to three oxygen atoms. It is single bonded to two of these oxygen atoms, each of which has three lone pairs of electrons, and double bonded to the third, which has two lone pairs of electrons. The double bond is located between the lower left oxygen atom and the carbon atom. The central and right structures are the same as the first, but the position of the double bonded oxygen has moved to the lower right oxygen in the central structure and to the top oxygen in the right structure.

One oxygen atom must have a double bond to carbon to complete the octet on the central atom. All oxygen atoms, however, are equivalent, and the double bond could form from any one of the three atoms. This gives rise to three resonance forms of the carbonate ion. Because we can write three identical resonance structures, we know that the actual arrangement of electrons in the carbonate ion is the average of the three structures. Again, experiments show that all three C–O bonds are exactly the same.

Questions & Answers

what is hydrocarbon
Sensible Reply
this is an organic compound that consist of hydrogen and ox
Okafor
why is isotonic solution the as blood
Hawa Reply
Elaborate more pls
Adeyinka
could you rephrase the question please?
Elaine
what is hydrocarbon? pls
Sensible
Hydrocarbon are organic compound consisting of hydrogen and carbon atoms
Adeyinka
thanks
Sensible
why is electrolytes really active in water
Sensible
relationship between an orbit and a shell
Hawa Reply
relationship between orbit and a shell
Hawa
What a homogenous mixture
Allison Reply
these are mixture of same substance or particles
INETIABOR
they are mixture of same particles or substance
INETIABOR
Define homologous series
Chizoba Reply
it is an organic compound having the same chemical properties which differs in a successive member of CH4
Okafor
when iron is exposed to moisture and it rusts,the value of deltaG for the reaction is
Mary Reply
negative
Max
table
Witszy
what is chemistry
Siyanbola Reply
it is a branch of science that deals with the mixture of chemicals and properties and uses of matter
Okafor
define aci according to Lewis
Ryhanna Reply
an acid is an electron pair acceptor according to lewis
Mary
What is a homogenous mixture
Allison
it is d mixture of d same substances or particle or compound
Okafor
homogenous is the combination or mixture of the same particle or compound
johnson
the third ionization energy of aluminium
sanmi Reply
name the allotrope of carbon used in gas masks
Moses Reply
Discuss the synthesis of sucrose
Ojiya Reply
What is a glycoside and discuss the formation
Ojiya
explain the transitioning of glucose molecules from Fischer projection of glucose to the Haworth projection
Rosey Reply
what is quantum
hezekiah Reply
what are metalliod
hezekiah
Metalloids are substances having the characteristics of both metals and non-metals
Ngwesse
what is acid
Tobi
define acid according to lewis
Ryhanna
Acid is an electron acceptor
Adeyinka
Wat z quantum
Adeyinka
what is valency
odukoya Reply
The number of chemical bonds the atoms of a certain element can form.
kenicia
what is ionization energy?
Hussniz Reply
ionization energy is qualitatively defined as the minimum amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated neutral gaseous atom, molecule or io
Tobi
Wow. Thumb 👍 up
Adeyinka
Very explanatory. Tanx
Adeyinka
Practice Key Terms 5

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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