Electronegativity, Formal Charge, and Resonance (9.3.3) Flashcards
• Formal charge explains the extreme reactivity of some compounds.
• Formal charge explains the extreme reactivity of some compounds.
• Formal charge helps explain why some molecules are more stable with
expanded octets.
• Formal charge helps explain why some molecules are more stable with
expanded octets.
• Formal charge explains why some molecules are best described with incomplete
octets.
• Formal charge explains why some molecules are best described with incomplete
octets.
• Formal charge explains why a resonance stabilized molecule can be more acidic.
• Formal charge explains why a resonance stabilized molecule can be more acidic.
Formal charge explains the extreme reactivity of
some compounds. For example, because formal
charges greater than +1 or less than –1 are not
allowed, structure A is the only structure of fulminate
ion (CNO–
) that is observed. Since the formal
charges on nitrogen (+1) and carbon (–1) are
different from what they would prefer to be, the
fulminate ion is not very stable.
Formal charge helps explain why some molecules
are more stable with expanded octets. For
example, when sulfur dioxide (SO2) is written with
an expanded octet, the formal charges on all three
atoms are zero. Formal charges of zero are
preferred.
Formal charge explains why some molecules are
best described with incomplete octets. For
example, two Lewis dot structures can be written
for boron trifluoride (BF3). However, the structure
on the left has a +1 formal charge on fluorine. Since
fluorine has a high electronegativity, this formal
charge is not tolerated very well. Breaking the octet
rule for boron yields a structure with formal charges
of zero on all atoms.
Formal charge explains why a resonance stabilized
molecule can be more acidic. For example, ethanol
is not very acidic, but acetic acid is acidic. Acetic
acid is acidic because of the relative stability of the
anion formed when the hydrogen is removed. The
oxygen atom in the ethoxide anion (the ion formed
from the deprotonation of ethanol) has a formal
charge of –1. However, since the acetate anion (the
ion formed from the deprotonation of acetic acid)
has two resonance structures, the oxygen atoms
in the acetate anion both have a formal charge of
–1/2. Therefore, the acetate anion is more stable,
and acetic acid is more acidic.
Formal charge explains the extreme reactivity of
some compounds. For example, because formal
charges greater than +1 or less than –1 are not
allowed, structure A is the only structure of fulminate
ion (CNO–
) that is observed. Since the formal
charges on nitrogen (+1) and carbon (–1) are
different from what they would prefer to be, the
fulminate ion is not very stable.
Formal charge helps explain why some molecules
are more stable with expanded octets. For
example, when sulfur dioxide (SO2) is written with
an expanded octet, the formal charges on all three
atoms are zero. Formal charges of zero are
preferred.
Formal charge explains why some molecules are
best described with incomplete octets. For
example, two Lewis dot structures can be written
for boron trifluoride (BF3). However, the structure
on the left has a +1 formal charge on fluorine. Since
fluorine has a high electronegativity, this formal
charge is not tolerated very well. Breaking the octet
rule for boron yields a structure with formal charges
of zero on all atoms.
Formal charge explains why a resonance stabilized
molecule can be more acidic. For example, ethanol
is not very acidic, but acetic acid is acidic. Acetic
acid is acidic because of the relative stability of the
anion formed when the hydrogen is removed. The
oxygen atom in the ethoxide anion (the ion formed
from the deprotonation of ethanol) has a formal
charge of –1. However, since the acetate anion (the
ion formed from the deprotonation of acetic acid)
has two resonance structures, the oxygen atoms
in the acetate anion both have a formal charge of
–1/2. Therefore, the acetate anion is more stable,
and acetic acid is more acidic.
What is electronegativity?
Electronegativity is a quantitative measure of the tendency of an atom to attract electrons to itself. (C)
This is the correct definition for electronegativity. An atom that has a higher electronegativity will exert a greater pull on electrons from another element in the formation of a bond.
The azide anion, [ N3 ]−, is used in air bags to trigger inflation upon impact. It is used because it is an unstable anion that readily decomposes into nitrogen gas (N2 ). Which of the following correctly rationalizes the azide ion’s unstable nature?
All of the nitrogen atoms would prefer to have a formal charge of 0 or −1. (A)
This is true. Since you are told that nitrogen has a relatively high electronegativity, each nitrogen atom will prefer a formal charge of 0 (the “best” electron arrangement, since an octet is created) or −1.
Suppose you have a molecule which contains a carbon-oxygen (C–O) bond. Suppose also that there are three resonance structures for this molecule. One resonance structure has a C–O single bond. The other one has a C–O double bond, and the third structure has a C–O triple bond. You observe in experiments that the distance between the carbon and oxygen atoms is closest to the C–O double bond resonance structure.
Which statement best describes the significant contributions of the different resonance structures?
The C–O single and triple bond configurations probably contribute less than the C–O double bond configuration to the arrangement. (B)
This is true and is verified by the distance observations.
Scientists use electronegativity values of elements to predict how elements will react. Which of the following statements is not true about an atom of an element as a component of a molecule having resonance structures?
If the element is highly electronegative, the atom will likely have a formal charge of +1 in a stable resonance structure. (C)
Which statement is not a possible explanation for behavior that violates the octet rule?
Atoms will always try to maximize their formal charge. (D)
This is false. Atoms never want to maximize their formal charges. Their nature is actually to assume a charge that is as close to 0 as possible.
Which of the following statements is not correct?
An atom with a low electronegativity and a formal charge of 0 will have a tendency to acquire another electron. (D)
This is not true because an atom with a low electronegativity will have a tendency to avoid acquisition of another electron, no matter what its formal charge is.
Acetate anion is very stable because its formal charge of −1 is shared equally among two oxygen atoms in the two resonance structures. This situation results in what defining characteristic for acetic acid?
acidity (B)
When the formal charge is equally distributed among the two oxygen sites, the result is a stable acetate anion with a characteristic acidity.
Which of the following is not a principal characteristic of atoms and molecules that scientists use to predict chemical behavior?
chemical use of a substance (D)
The use of a chemical, including both its application and frequency of application, is dependent upon the substance’s chemical characteristics.
Molecules can arrange themselves in ways that violate the octet rule of having eight electrons around each atom. What occurs is a more stable electron arrangement, in which atoms can have more than eight or less than eight electrons. Which of the following best explains why this occurs?
The molecule is trying to assume an electron arrangement in which all the atoms have a formal charge as close as possible (or equal) to zero. (C)
Sometimes, in order to stabilize each atom, a molecule will violate the octet rule so that the formal charges of each contributing atom are as close (or equal) to 0 as possible.
