9.3.2 Formal Charge

Question 1

Formal Charge

Answer 1

• The more favorable resonance structures can be determined using formal charge.
• A formal charge of zero is most stable; values of +1 or –1 are acceptable; any other value will not make a significant contribution to the resonance structure.
• The connectivity of the atoms can be established by determining the configuration of atoms with the lowest formal charge.

Question 2

note

Answer 2

• The more favorable resonance structures can be determined using formal charge.
• To assign formal charges to the atoms in a molecule, begin by assigning all of the lone pairs and half of the bonding electrons to the atom to which they are attached.
• Calculate formal charge for each atom as the number of valence electrons in the neutral atom minus the number of electrons assigned in step 1. The example shows the formal charge for the first nitrogen is 0, for the second nitrogen is +1, and for the oxygen is –1.
• To determine the more favorable resonance structures a formal charge of zero is most stable; values of +1 or –1 are acceptable; any other value will not make a significant contribution to the resonance structure.
• In the example, since the first nitrogen in the lower resonance structure has a formal charge of –2, this structure will not contribute significantly to the structure of nitrous oxide. The other two resonance structures have acceptable formal charges and will contribute significantly to the overall structure.
• The connectivity of the atoms can be established by
  determining the configuration of atoms with the lowest formal charge.
• In the example, the right-hand structure for CO 2 has formal charge on two atoms of +2 and –2 indicating that this structure does not contribute significantly to the structure of the molecule. The molecule can be adequately described using the connectivity of atoms shown on the left side.

Question 3

What is the formal charge of S in the Lewis dot structure shown above?

Answer 3

0

Question 4

Why are resonance structures useful for some Lewis dot structures?

Answer 4

• Resonance structures can provide an explanation for molecules having more than one valid Lewis dot structure.
• Resonance structures contribute to the bonding (models) for a molecule or molecule ion.
• Resonance structures, along with formal charges, help you to better understand how the arrangement of electrons will affect a molecule’s overall properties.

Question 5

What are the formal charges on the atoms in sulfur dioxide, shown below?

Answer 5

left oxygen: −1; sulfur: +1; right oxygen: 0

Question 6

Ozone has two resonance structures. Which statement is not true about the resonance structures for ozone?

Answer 6

The resonance structures do not suggest the electrons in the bonds are being shared

Question 7

Find the formal charges for each atom in this carbon monoxide molecule. (The number of valence electrons for carbon is 4 and for oxygen is 6.)

Answer 7

carbon = −1; oxygen = +1

Question 8

Which of the following statements is false?

Answer 8

Resonance structures, by nature, are always equivalent.

Question 9

In one particular resonance structure for nitrous oxide, one of the nitrogen atoms has the following, unique electron distribution:

Why is this particular nitrous oxide resonance structure less significant than the other nitrous oxide resonance structures?

Answer 9

It has an electron distribution that is trying to put a lot of electrons around the left-most nitrogen atom at the expense of the other atoms.

Question 10

What are the two main steps for calculating the formal charge for each atom? You can assume that you have already drawn the correct Lewis dot structure for the molecule.

Answer 10

1) Assign all of the lone pairs to their atom and 1/2 of the bonding electrons to that attached atom. 2) Subtract the number of Lewis dot structure electrons assigned to the atom from the number of valence electrons.

Question 11

Formal charge provides a way to distinguish between different resonance structures of the same molecule. Suppose that you have drawn all the possible resonance structures for a molecule. What is the first main step in calculating the formal charge?

Answer 11

Assign all of the lone pairs and one-half of the bonding electrons to the atom to which they are attached.

Question 12

What is a resonance structure?

Answer 12

A resonance structure is a hybrid of two or more Lewis dot structures that both contribute to the description of the bonding in a molecule.

Question 13

Which of the following statements about formal charge is true?

Answer 13

You can use formal charges to rank the relative importance of different resonance structures of a molecule.