9.3.3 Electronegativity, Formal Charge, and Resonance

Question 1

Electronegativity, Formal Charge, and Resonance

Answer 1

• Formal charge explains the extreme reactivity of some compounds.
• 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 a resonance stabilized molecule can be more acidic.

Question 2

note

Answer 2

• 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 (SO 2 ) 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 (BF 3 ). 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.

Question 3

Magnesium (of Group 2(IIA) of the periodic table) has two valence electrons. Its atoms attain the (octet) electron configuration of noble gas neon when they each lose their two valence electrons and form the cation, magnesium ion.
Mg -> Mg2+ + 2e-
Which of the following statements about magensium’s electronegativity is most accurate?

Answer 3

Magnesium has a relatively low electronegativity. Magnesium tends to lose two valence electrons in order to assume an octet arrangement.

Question 4

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?

Answer 4

acidity

Question 5

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?

Answer 5

The C–O single and triple bond configurations probably contribute less than the C–O double bond configuration to the arrangement.

Question 6

Which statement is not a possible explanation for behavior that violates the octet rule?

Answer 6

Atoms will always try to maximize their formal charge.

Question 7

Which of the following is not a principal characteristic of atoms and molecules that scientists use to predict chemical behavior?

Answer 7

chemical use of a substance

Question 8

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?

Answer 8

All of the nitrogen atoms would prefer to have a formal charge of 0 or −1.

Question 9

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?

Answer 9

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.

Question 10

What is electronegativity?

Answer 10

Electronegativity is a quantitative measure of the tendency of an atom to attract electrons to itself.

Question 11

Which of the following statements is not correct?

Answer 11

An atom with a low electronegativity and a formal charge of 0 will have a tendency to acquire another electron.

Question 12

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?

Answer 12

If the element is highly electronegative, the atom will likely have a formal charge of +1 in a stable resonance structure.