Why Do We Calculate Formal Charges When Drawing Lewis Structures?

Affiliate vii. Chemic Bonding and Molecular Geometry

vii.4 Formal Charges and Resonance

Learning Objectives

Past the end of this department, you volition be able to:

• Compute formal charges for atoms in any Lewis structure
• Use formal charges to identify the most 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 nosotros accept seen, however, in some cases, there is seemingly more than than one valid structure for a molecule. We can use the concept of formal charges to help us predict the most appropriate Lewis structure when more i is reasonable.

Calculating Formal Charge

The formal charge of an cantlet in a molecule is the hypothetical charge the cantlet would accept if we could redistribute the electrons in the bonds evenly between the atoms. Another way of saying this is that formal accuse results when we take the number of valence electrons of a neutral atom, subtract the nonbonding electrons, and then subtract the number of bonds connected to that atom in the Lewis structure.

Thus, we summate formal charge as follows:

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

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

We must call up that the formal charge calculated for an atom is not the actual charge of the atom in the molecule. Formal charge is just a useful bookkeeping procedure; it does not point the presence of actual charges.

Example 1

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

Solution

1. We separate the bonding electron pairs equally for all I–Cl bonds:
   
2. We assign alone pairs of electrons to their atoms. Each Cl atom now has vii electrons assigned to it, and the I cantlet has eight.
3. Subtract this number from the number of valence electrons for the neutral atom: I: 7 – 8 = –1Cl: 7 – 7 = 0The sum of the formal charges of all the atoms equals –one, which is identical to the charge of the ion (–ane).

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

Case two

Calculating Formal Accuse 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 atom and one to the Cl atom in that bond:
   
2. Assign the lone pairs to their cantlet. 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 – seven = 0Cl: 7 – 7 = 0

   All atoms in BrCl_iii have a formal charge of zero, and the sum of the formal charges totals nil, as it must in a neutral molecule.

Check Your Learning
Determine the formal charge for each cantlet in NCl_three.

Answer:

Northward: 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, post-obit the steps for writing Lewis structures may pb to more than one possible molecular construction—different multiple bond and alone-pair electron placements or different arrangements of atoms, for instance. A few guidelines involving formal charge can be helpful in deciding which of the possible structures is virtually likely for a item molecule or ion:

1. A molecular structure in which all formal charges are zero is preferable to i in which some formal charges are not zippo.
2. If the Lewis structure must accept 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 opposite sign.
4. When we must choose amidst several Lewis structures with similar distributions of formal charges, the construction with the negative formal charges on the more than electronegative atoms is preferable.

To run into how these guidelines utilize, permit us consider some possible structures for carbon dioxide, CO₂. We know from our previous word that the less electronegative atom typically occupies the cardinal position, but formal charges let us to understand why this occurs. We can draw three possibilities for the construction: carbon in the center and double bonds, carbon in the center with a single and triple bond, and oxygen in the center with double bonds:

Comparing the three formal charges, we tin definitively place the structure on the left as preferable because it has only formal charges of null (Guideline 1).

As some other example, the thiocyanate ion, an ion formed from a carbon atom, a nitrogen atom, and a sulfur atom, could take three unlike molecular structures: CNS^–, NCS^–, or CSN^–. The formal charges nowadays in each of these molecular structures tin can assist usa option the most likely system of atoms. Possible Lewis structures and the formal charges for each of the 3 possible structures for the thiocyanate ion are shown here:

Notation that the sum of the formal charges in each case is equal to the accuse of the ion (–1). However, the start arrangement of atoms is preferred because it has the everyman number of atoms with nonzero formal charges (Guideline two). Also, it places the least electronegative cantlet in the center, and the negative charge on the more electronegative element (Guideline 4).

Example 3

Using Formal Charge to Decide Molecular Structure
Nitrous oxide, N_twoO, unremarkably known every bit laughing gas, is used as an anesthetic in pocket-sized surgeries, such every bit the routine extraction of wisdom teeth. Which is the likely construction for nitrous oxide?

Solution
Determining formal charge yields the following:

The structure with a final oxygen atom all-time satisfies the criteria for the most stable distribution of formal charge:

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

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 Example iii tin can have two possible structures with the atoms in the same positions. The electrons involved in the Due north–O double bond, nevertheless, are in different positions:

If nitrite ions exercise indeed contain a unmarried and a double bond, we would wait for the two bond lengths to be unlike. A double bond between two atoms is shorter (and stronger) than a single bond between the same two atoms. Experiments show, notwithstanding, that both Northward–O bonds in NO₂ ⁻ take the same strength and length, and are identical in all other properties.

It is non possible to write a unmarried Lewis structure for NO₂ ⁻ in which nitrogen has an octet and both bonds are equivalent. Instead, nosotros use the concept of resonance: if two or more Lewis structures with the same arrangement of atoms tin be written for a molecule or ion, the actual distribution of electrons is an average of that shown past the various Lewis structures. The actual distribution of electrons in each of the nitrogen-oxygen bonds in NO_ii ⁻ is the average of a double bond and a unmarried bond. We phone 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 private resonance forms. A double-headed arrow between Lewis structures indicates that they are resonance forms. Thus, the electronic structure of the NO₂ ⁻ ion is shown every bit:

Nosotros should recall that a molecule described as a resonance hybrid never possesses an electronic structure described past either resonance form. It does not fluctuate between resonance forms; rather, the bodily electronic structure is always the boilerplate of that shown past all resonance forms. George Wheland, one of the pioneers of resonance theory, used a historical illustration to draw the relationship between resonance forms and resonance hybrids. A medieval traveler, having never before seen a rhinoceros, described it equally 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 be. Information technology has some characteristics in mutual with its resonance forms, only the resonance forms themselves are convenient, imaginary images (like the unicorn and the dragon).

The carbonate anion, CO₃ ²⁻, provides a second example of resonance:

Ane oxygen cantlet must have a double bond to carbon to complete the octet on the central atom. All oxygen atoms, notwithstanding, are equivalent, and the double bail could form from any i of the iii atoms. This gives rise to three resonance forms of the carbonate ion. Because we tin write three identical resonance structures, nosotros know that the bodily system of electrons in the carbonate ion is the average of the 3 structures. Over again, experiments show that all three C–O bonds are exactly the same.

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

Key Concepts and Summary

In a Lewis construction, formal charges can be assigned to each atom by treating each bond equally if one-half of the electrons are assigned to each atom. These hypothetical formal charges are a guide to determining the most appropriate Lewis structure. A structure in which the formal charges are as close to zero as possible is preferred. Resonance occurs in cases where two or more Lewis structures with identical arrangements of atoms only dissimilar distributions of electrons can exist written. The actual distribution of electrons (the resonance hybrid) is an boilerplate of the distribution indicated past the individual Lewis structures (the resonance forms).

Central Equations

• [latex]\text{formal accuse} = \# \;\text{valence trounce electrons (complimentary atom)} \; - \;\# \;\text{lone pair electrons}\; - \frac{1}{2} \# \;\text{bonding electrons}[/latex]

Chemical science Terminate of Affiliate Exercises

1. Write resonance forms that describe 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 two O atoms)

   (d) benzene, C₆H₆:

   

   (east) the formate ion:

   

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

   (a) sulfur dioxide, And then_ii

   (b) carbonate ion, CO₃ ²⁻

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

   (d) pyridine:

   

   (due east) the allyl ion:

   

3. Write the resonance forms of ozone, O₃, the component of the upper atmosphere that protects the Globe from ultraviolet radiations.
4. Sodium nitrite, which has been used to preserve bacon and other meats, is an ionic chemical compound. Write the resonance forms of the nitrite ion, NO₂ ^–.
5. In terms of the bonds nowadays, explain why acerb acid, CH₃CO₂H, contains two singled-out types of carbon-oxygen bonds, whereas the acetate ion, formed by loss of a hydrogen ion from acerb acrid, simply contains one type of carbon-oxygen bail. The skeleton structures of these species are shown:
   
6. Write the Lewis structures for the following, and include resonance structures where appropriate. Indicate which has the strongest carbon-oxygen bond.

   (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 appropriate.
8. Determine the formal charge of each element in the following:

   (a) HCl

   (b) CF₄

   (c) PCl_iii

   (d) PF_v

9. Decide the formal accuse of each element in the following:

   (a) H₃O⁺

   (b) SO₄ ⁱⁱ⁻

   (c) NH₃

   (d) O₂ ²⁻

   (e) H₂O₂

10. Summate the formal charge of chlorine in the molecules Cl₂, BeCl_ii, and ClF₅.
11. Summate the formal charge of each element in the following compounds and ions:

    (a) F₂CO

    (b) NO^–

    (c) BF₄ ⁻

    (d) SnCl_three ⁻

    (eastward) H₂CCH₂

    (f) ClF₃

    (one thousand) SeF_half-dozen

    (h) PO₄ ³⁻

12. Describe all possible resonance structures for each of these compounds. Determine the formal charge on each atom in each of the resonance structures:

    (a) O₃

    (b) And then₂

    (c) NO₂ ⁻

    (d) NO_iii ⁻

13. Based on formal charge considerations, which of the following would probable be the correct arrangement of atoms in nitrosyl chloride: ClNO or ClON?
14. Based on formal accuse considerations, which of the following would probable exist the correct organization of atoms in hypochlorous acid: HOCl or OClH?
15. Based on formal accuse considerations, which of the following would probable be the right arrangement of atoms in sulfur dioxide: OSO or SOO?
16. Draw the structure of hydroxylamine, H_threeNO, and assign formal charges; look upward the structure. Is the actual structure consistent with the formal charges?
17. Iodine forms a serial of fluorides (listed hither). Write Lewis structures for each of the iv compounds and make up one's mind the formal accuse of the iodine atom in each molecule:

    (a) IF

    (b) IF₃

    (c) IF₅

    (d) IF₇

18. Write the Lewis structure and chemical formula of the compound with a molar mass of about lxx g/mol that contains nineteen.seven% nitrogen and fourscore.3% fluorine by mass, and determine the formal charge of the atoms in this compound.
19. Which of the post-obit structures would we expect for nitrous acrid? Decide the formal charges:
    
20. Sulfuric acrid is the industrial chemical produced in greatest quantity worldwide. About 90 billion pounds are produced each year in the United States lone. Write the Lewis structure for sulfuric acrid, H₂So₄, which has two oxygen atoms and two OH groups bonded to the sulfur.

Glossary

formal charge

charge that would result on an atom by taking the number of valence electrons on the neutral atom and subtracting the nonbonding electrons and the number of bonds (one-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 than Lewis structures that have the same arrangement of atoms but different arrangements of electrons

resonance hybrid

average of the resonance forms shown by the individual Lewis structures

Solutions

Answers to Chemical science End of Chapter Exercises

2. (a)

(b)

(c)

(d)

(e)

4.

half dozen. (a)

(b)

CO has the strongest carbon-oxygen bond considering there is a triple bail joining C and O. CO₂ has double bonds.

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

ten. Cl in Cl_two: 0; Cl in BeCl_two: 0; Cl in ClF₅: 0

12. (a)
;

(b)
;

(c)
;

(d)

14. HOCl

sixteen. The construction that gives zero formal charges is consequent with the bodily construction:

18. NF_three;

20.

Source: https://opentextbc.ca/chemistry/chapter/7-4-formal-charges-and-resonance/

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