The Following Three Lewis Structures Can Be Drawn for N2o:

Affiliate 7. Chemical Bonding and Molecular Geometry

vii.4 Formal Charges and Resonance

Learning Objectives

By the finish of this department, you volition be able to:

• Compute formal charges for atoms in any Lewis structure
• Use formal charges to place the nearly reasonable Lewis structure for a given molecule
• Explain the concept of resonance and draw Lewis structures representing resonance forms for a given molecule

In the previous section, we discussed how to write Lewis structures for molecules and polyatomic ions. As we accept seen, still, in some cases, at that place is seemingly more than one valid structure for a molecule. We can utilize the concept of formal charges to help us predict the most appropriate Lewis structure when more than one is reasonable.

Calculating Formal Charge

The formal charge of an atom in a molecule is the hypothetical charge the atom would have if we could redistribute the electrons in the bonds evenly between the atoms. Some other style of saying this is that formal charge results when we take the number of valence electrons of a neutral atom, subtract the nonbonding electrons, and so decrease the number of bonds connected to that atom in the Lewis structure.

Thus, we calculate formal charge as follows:

[latex]\text{formal charge} = \# \;\text{valence shell electrons (free atom)} \; - \;\# \;\text{lone pair electrons}\; - \frac{ane}{2} \# \;\text{bonding electrons}[/latex]

We can double-check formal accuse calculations past determining the sum of the formal charges for the whole structure. The sum of the formal charges of all atoms in a molecule must be zero; the sum of the formal charges in an ion should equal the charge of the ion.

We must remember that the formal charge calculated for an atom is non the actual charge of the cantlet in the molecule. Formal charge is simply a useful bookkeeping procedure; it does not indicate the presence of bodily charges.

Example 1

Calculating Formal Charge from Lewis Structures
Assign formal charges to each atom in the interhalogen ion ICl₄ ⁻.

Solution

1. We divide the bonding electron pairs as for all I–Cl bonds:
   
2. We assign alone pairs of electrons to their atoms. Each Cl cantlet at present has vii electrons assigned to it, and the I atom has eight.
3. Decrease this number from the number of valence electrons for the neutral atom: I: 7 – viii = –1Cl: seven – 7 = 0The sum of the formal charges of all the atoms equals –1, which is identical to the charge of the ion (–1).

Check Your Learning
Summate the formal charge for each atom in the carbon monoxide molecule:

Example 2

Calculating Formal Charge from Lewis Structures
Assign formal charges to each cantlet in the interhalogen molecule BrCl₃.

Solution

1. Assign one of the electrons in each Br–Cl bond to the Br cantlet and one to the Cl atom in that bond:
   
2. Assign the lone pairs to their atom. Now each Cl atom has seven electrons and the Br atom has seven electrons.
3. Subtract this number from the number of valence electrons for the neutral atom. This gives the formal charge:Br: 7 – 7 = 0Cl: 7 – 7 = 0

   All atoms in BrCl₃ take a formal charge of cypher, and the sum of the formal charges totals zero, as it must in a neutral molecule.

Bank check Your Learning
Determine the formal charge for each cantlet in NCl₃.

Answer:

N: 0; all three Cl atoms: 0

Using Formal Charge to Predict Molecular Structure

The arrangement of atoms in a molecule or ion is called its molecular structure. In many cases, following the steps for writing Lewis structures may pb to more than than one possible molecular structure—dissimilar multiple bond and lone-pair electron placements or unlike arrangements of atoms, for instance. A few guidelines involving formal accuse tin can be helpful in deciding which of the possible structures is most likely for a detail molecule or ion:

1. A molecular structure in which all formal charges are nix is preferable to 1 in which some formal charges are non zero.
2. If the Lewis construction must have nonzero formal charges, the arrangement with the smallest nonzero formal charges is preferable.
3. Lewis structures are preferable when adjacent formal charges are zero or of the reverse sign.
4. When we must choose amid several Lewis structures with similar distributions of formal charges, the structure with the negative formal charges on the more than electronegative atoms is preferable.

To come across how these guidelines apply, allow us consider some possible structures for carbon dioxide, CO₂. We know from our previous discussion that the less electronegative cantlet typically occupies the primal position, but formal charges let us to understand why this occurs. We tin can draw three possibilities for the structure: carbon in the center and double bonds, carbon in the center with a unmarried and triple bail, and oxygen in the center with double bonds:

Comparing the 3 formal charges, nosotros can definitively identify the construction on the left equally preferable because it has but formal charges of zero (Guideline 1).

As another example, the thiocyanate ion, an ion formed from a carbon atom, a nitrogen atom, and a sulfur atom, could accept three different molecular structures: CNS^–, NCS^–, or CSN^–. The formal charges nowadays in each of these molecular structures can help u.s. pick the well-nigh likely arrangement of atoms. Possible Lewis structures and the formal charges for each of the iii possible structures for the thiocyanate ion are shown here:

Notation that the sum of the formal charges in each case is equal to the charge of the ion (–ane). However, the first arrangement of atoms is preferred because it has the everyman number of atoms with nonzero formal charges (Guideline 2). As well, information technology places the least electronegative atom in the center, and the negative charge on the more than electronegative chemical element (Guideline 4).

Example 3

Using Formal Charge to Determine Molecular Construction
Nitrous oxide, N₂O, usually known as laughing gas, is used as an anesthetic in pocket-size surgeries, such equally the routine extraction of wisdom teeth. Which is the likely structure for nitrous oxide?

Solution
Determining formal charge yields the post-obit:

The structure with a terminal oxygen cantlet best satisfies the criteria for the most stable distribution of formal charge:

The number of atoms with formal charges are minimized (Guideline ii), and there is no formal charge larger than 1 (Guideline 2). This is once more consistent with the preference for having the less electronegative atom in the key position.

Bank check Your Learning
Which is the most likely molecular structure for the nitrite (NO₂ ⁻) ion?

Resonance

You may have noticed that the nitrite anion in Instance 3 tin can have 2 possible structures with the atoms in the aforementioned positions. The electrons involved in the N–O double bail, nonetheless, are in different positions:

If nitrite ions exercise indeed contain a single and a double bond, we would expect for the ii bail lengths to be different. A double bond between 2 atoms is shorter (and stronger) than a single bail betwixt the same two atoms. Experiments testify, withal, that both N–O bonds in NO₂ ⁻ accept the same force and length, and are identical in all other properties.

Information technology is not possible to write a single Lewis structure for NO_ii ⁻ in which nitrogen has an octet and both bonds are equivalent. Instead, we utilise the concept of resonance: if 2 or more Lewis structures with the same system of atoms can be written for a molecule or ion, the bodily distribution of electrons is an average of that shown by the diverse Lewis structures. The actual distribution of electrons in each of the nitrogen-oxygen bonds in NO₂ ⁻ is the average of a double bond and a unmarried bail. We call the individual Lewis structures resonance forms. The actual electronic construction of the molecule (the average of the resonance forms) is called a resonance hybrid of the private resonance forms. A double-headed pointer between Lewis structures indicates that they are resonance forms. Thus, the electronic structure of the NO₂ ⁻ ion is shown as:

Nosotros should remember that a molecule described as a resonance hybrid never possesses an electronic construction described past either resonance class. Information technology does non fluctuate betwixt resonance forms; rather, the bodily electronic construction 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 draw the human relationship between resonance forms and resonance hybrids. A medieval traveler, having never before seen a rhino, described information technology every bit a hybrid of a dragon and a unicorn because it had many backdrop in common with both. Just as a rhino 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 rhino, it is a existent 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, CO₃ ²⁻, provides a second instance of resonance:

Ane oxygen atom must have a double bail to carbon to complete the octet on the key cantlet. All oxygen atoms, however, are equivalent, and the double bond could grade from whatsoever 1 of the three atoms. This gives rise to three resonance forms of the carbonate ion. Because nosotros tin 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 evidence that all 3 C–O bonds are exactly the same.

The online Lewis Construction Make includes many examples to practice drawing resonance structures.

Primal Concepts and Summary

In a Lewis structure, formal charges can exist assigned to each atom by treating each bond as if half of the electrons are assigned to each atom. These hypothetical formal charges are a guide to determining the about appropriate Lewis structure. A structure in which the formal charges are as close to zippo as possible is preferred. Resonance occurs in cases where ii or more than Lewis structures with identical arrangements of atoms but unlike distributions of electrons can be written. The actual distribution of electrons (the resonance hybrid) is an average of the distribution indicated past the individual Lewis structures (the resonance forms).

Key Equations

• [latex]\text{formal charge} = \# \;\text{valence shell electrons (free atom)} \; - \;\# \;\text{alone pair electrons}\; - \frac{1}{two} \# \;\text{bonding electrons}[/latex]

Chemistry End of Chapter Exercises

1. Write resonance forms that depict the distribution of electrons in each of these molecules or ions.

   (a) selenium dioxide, OSeO

   (b) nitrate ion, NO₃ ⁻

   (c) nitric acid, HNO_three (N is bonded to an OH group and ii O atoms)

   (d) benzene, C_half-dozenH₆:

   

   (e) the formate ion:

   

2. Write resonance forms that draw the distribution of electrons in each of these molecules or ions.

   (a) sulfur dioxide, And then₂

   (b) carbonate ion, CO_iii ²⁻

   (c) hydrogen carbonate ion, HCO₃ ⁻ (C is bonded to an OH grouping and two O atoms)

   (d) pyridine:

   

   (eastward) the allyl ion:

   

3. Write the resonance forms of ozone, O_three, the component of the upper atmosphere that protects the Earth from ultraviolet radiations.
4. Sodium nitrite, which has been used to preserve bacon and other meats, is an ionic compound. Write the resonance forms of the nitrite ion, NO₂ ^–.
5. In terms of the bonds present, explain why acetic acid, CH₃CO₂H, contains two singled-out types of carbon-oxygen bonds, whereas the acetate ion, formed by loss of a hydrogen ion from acetic acid, but contains one blazon of carbon-oxygen bond. The skeleton structures of these species are shown:
   
6. Write the Lewis structures for the following, and include resonance structures where appropriate. Bespeak which has the strongest carbon-oxygen bail.

   (a) CO₂

   (b) CO

7. Toothpastes containing sodium hydrogen carbonate (sodium bicarbonate) and hydrogen peroxide are widely used. Write Lewis structures for the hydrogen carbonate ion and hydrogen peroxide molecule, with resonance forms where advisable.
8. Determine the formal charge of each element in the following:

   (a) HCl

   (b) CF₄

   (c) PCl₃

   (d) PF₅

9. Decide the formal charge of each chemical element in the following:

   (a) H_threeO⁺

   (b) And so₄ ^two−

   (c) NH₃

   (d) O₂ ⁱⁱ⁻

   (e) H_twoO_two

10. Calculate the formal charge of chlorine in the molecules Cl_ii, BeCl₂, and ClF₅.
11. Calculate the formal charge of each element in the post-obit compounds and ions:

    (a) F₂CO

    (b) NO^–

    (c) BF₄ ⁻

    (d) SnCl_iii ⁻

    (due east) H_twoCCH_two

    (f) ClF₃

    (g) SeF₆

    (h) PO_four ⁱⁱⁱ⁻

12. Draw all possible resonance structures for each of these compounds. Make up one's mind the formal accuse on each atom in each of the resonance structures:

    (a) O_three

    (b) And so_two

    (c) NO₂ ⁻

    (d) NO₃ ⁻

13. Based on formal charge considerations, which of the post-obit would likely be the correct organization of atoms in nitrosyl chloride: ClNO or ClON?
14. Based on formal accuse considerations, which of the post-obit would likely be the correct arrangement of atoms in hypochlorous acrid: HOCl or OClH?
15. Based on formal accuse considerations, which of the following would probable be the correct arrangement of atoms in sulfur dioxide: OSO or SOO?
16. Draw the construction of hydroxylamine, H₃NO, and assign formal charges; look up the construction. Is the bodily structure consistent with the formal charges?
17. Iodine forms a serial of fluorides (listed here). Write Lewis structures for each of the iv compounds and determine the formal charge of the iodine cantlet in each molecule:

    (a) IF

    (b) IF_three

    (c) IF₅

    (d) IF₇

18. Write the Lewis structure and chemical formula of the compound with a molar mass of nigh 70 g/mol that contains nineteen.seven% nitrogen and fourscore.3% fluorine by mass, and decide the formal charge of the atoms in this compound.
19. Which of the following structures would we expect for nitrous acid? Make up one's mind the formal charges:
    
20. Sulfuric acid is the industrial chemical produced in greatest quantity worldwide. About 90 billion pounds are produced each twelvemonth in the Usa alone. Write the Lewis construction for sulfuric acid, H₂Then_four, which has two oxygen atoms and 2 OH groups bonded to the sulfur.

Glossary

formal charge

charge that would consequence on an cantlet by taking the number of valence electrons on the neutral cantlet and subtracting the nonbonding electrons and the number of bonds (half of the bonding electrons)

molecular structure

arrangement of atoms in a molecule or ion

resonance

situation in which i Lewis structure is insufficient to describe the bonding in a molecule and the average of multiple structures is observed

resonance forms

2 or more Lewis structures that have the same organization of atoms but different arrangements of electrons

resonance hybrid

average of the resonance forms shown by the individual Lewis structures

Solutions

Answers to Chemistry Finish of Chapter Exercises

2. (a)

(b)

(c)

(d)

(e)

4.

6. (a)

(b)

CO has the strongest carbon-oxygen bond because there is a triple bond joining C and O. CO_ii has double bonds.

8. (a) H: 0, Cl: 0; (b) C: 0, F: 0; (c) P: 0, Cl 0; (d) P: 0, F: 0

10. Cl in Cl_ii: 0; Cl in BeCl₂: 0; Cl in ClF₅: 0

12. (a)
;

(b)
;

(c)
;

(d)

14. HOCl

xvi. The structure that gives zero formal charges is consequent with the actual construction:

18. NF_iii;

20.

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