Formal Charge (9.3.2) Flashcards
• The more favorable resonance structures can be determined using formal
charge.
• 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.
• 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.
• The connectivity of the atoms can be established by determining the configuration
of atoms with the lowest formal charge.
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 CO2 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.
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 CO2 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.
Which of the following statements about formal charge is true?
You can use formal charges to rank the relative importance of different resonance structures of a molecule. (C)
What is the formal charge of S in the Lewis dot structure shown above?
0 (B)
What are the formal charges on the atoms in sulfur dioxide, shown below?
left oxygen: −1; sulfur: +1; right oxygen: 0 (B)
Why are resonance structures useful for some Lewis dot structures?
All of the above. (D)
All of the answers are good explanations.
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.
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. (C)
This is the correct twostep process to find the formal charge for each atom.
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?
Assign all of the lone pairs and one-half of the bonding electrons to the atom to which they are attached. (B)
This is exactly what you should do for the first step. Looking at one resonance structure, you must assign each electron an atom. That is, all of the electrons must be accounted for. With shared electrons, you assign half to one atom and half to the other.
What is a resonance structure?
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. (B)
This is a very general (but accurate) definition of a resonance structure. The bonding between atoms in a molecule may be correctly represented in different ways, even though the connectivity of the atoms stays the same. Resonance structures allow you to pictorially represent these different bonding possibilities. The permutations give you a more informed picture of the actual bonding of the molecule.
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.)
:C:::O:
carbon = −1; oxygen = +1 (C)
To find this answer, let’s first look at the carbon atom. It has 6 electrons that it shares with oxygen. Therefore, half of those 6 electrons can be assigned to the carbon. Therefore, carbon has a total of 2 (the lone pair) + 1/2 (6) = 5 Lewis dot structure electrons. Thus, the formal charge of carbon = 4 − 5 = −1. Oxygen shares 6 electrons with the carbon atom, so half of those (1/2 (6)) can be assigned to the oxygen atom. So the oxygen atom has a total of (1/2) (6) + 2 = 5 Lewis dot structure electrons. The formal charge, is 6 − 5 = +1.
Ozone has two resonance structures. Which statement is not true about the resonance structures for ozone?
The resonance structures do not suggest the electrons in the bonds are being shared. (C)
They very much demonstrate the sharing of electrons between atoms. They show how the electrons can be arranged and shared in different ways.
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?
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. (B)
This situation causes the overall contribution of this particular resonance structure to be minimized.
Which of the following statements is false?
Resonance structures, by nature, are always equivalent. (D)
There are many examples of nonequivalent resonance structures. These nonequivalent structures illustrate that though there may be several different valid Lewis dot structures for a molecule, all the arrangements may not contribute equally to the overall behavior of the molecule.
