9.3.1 Resonance Structures

Question 1

Resonance Structures

Answer 1

• Resonance structures are needed when more than one Lewis dot structure is possible.
• Resonance structures can contribute equally or unequally to the overall structure of the molecule.

Question 2

note

Answer 2

• Resonance structures are needed when more than one Lewis dot structure is possible.
• In the example of ozone, the double bond can be between the central and the right-hand atom, or between the central and the left-hand atom. There is more than one valid Lewis dot structure for ozone. Each of these resonance structures satisfies the octet rule for each of the atoms but they predict an uneven geometry in the bonds.
• In fact, both structures contribute to the structure of the ozone molecule. Taken together they form a more complete model of the molecule than either would individually.
• Resonance structures can contribute equally or unequally.
• In the examples shown, the nitrate ion has three equivalent resonance structures that contribute equally to the overall structure of the ion. In contrast, the sulfur dioxide has two equivalent resonance structures and a third non-equivalent resonance structure. Nitrous oxide has three inequivalent resonance structures. The non-equivalent resonance structure contributes differently to the overall structure than the other two.
• In the case of sulfur dioxide, because of the ability of sulfur to have an expanded octet the third resonance structure is present when sulfur has ten electrons.
• In resonance structures the nuclei do not change position, nor do the connections between nuclei change; only the electrons move around.

Question 3

Which structure of the sulfate anion, [SO4 ]2−, is not equivalent to the other three?

Answer 3

This structure has only 8 octet electrons drawn for the sulfur atom, whereas the other structures have 12. This structure also has only single bonds between the sulfur atom and the oxygen atoms, whereas the other structures have double bonds between the sulfur atom and 2 of the oxygen atoms.

Question 4

Which of the following is not a resonance structure for the azide anion, [ N3 ]−?

Answer 4

Although the correct total number of valence electrons is drawn, the Lewis dot structure is incorrect because the nitrogen atom that is bonded to the other 2 nitrogen atoms has 12 valence electrons instead of 8.

Question 5

Which structure of phosphoric acid, H3PO4, is not equivalent to the other three?

Answer 5

This structure has only 8 octet electrons drawn for the phosphorus atom, whereas the other structures have 10. This structure also has only single bonds between the phosphorus atom and the oxygen atoms, whereas the other structures have a double bond between the phosphorus atom and one of the oxygen atoms.

Question 6

Which of the following is not a reasonable resonance structure for the [ NCO]− anion in which the carbon atom is the central atom?

Answer 6

This is not a reasonable resonance structure for an [ NCO]− anion in which the carbon atom is the central atom. The order of the atoms was changed so that the oxygen atom is the central atom.

Question 7

Which structure of sulfuric acid, H2SO4, is not equivalent to the other three?

Answer 7

This structure has only 8 octet electrons drawn for the sulfur atom, whereas the other structures have 12. This structure also has only single bonds between the sulfur atom and the oxygen atoms, whereas the other structures have double bonds between the sulfur atom and 2 of the oxygen atoms.

Question 8

Which structure of the phosphite anion, [PO3 ]3−, is not equivalent to the other three?

Answer 8

This structure has only 8 octet electrons drawn for the phosphorus atom, whereas the other structures have 10. This structure also has only single bonds between the phosphorus atom and the oxygen atoms, whereas the other structures have a double bond between the phosphorus atom and one of the oxygen atoms.

Question 9

Which of the following is not a valid Lewis dot structure for hydrogen azide, HN3?

Answer 9

Although the correct total number of valence electrons is drawn, the Lewis dot structure is incorrect because the nitrogen atom that is bonded to the other 2 nitrogen atoms has 12 valence electrons instead of 8.

Question 10

For which of the following molecules is it possible to draw resonance structures?

Answer 10

NO3−

Question 11

Which statement about resonance hybrids is correct?

Answer 11

A resonance hybrid structure describes a molecule’s structure better than any of the contributing resonance structures by itself.

Question 12

How many resonance structures can be drawn for a molecule whose structure is best explained by three different Lewis dot structures?

Answer 12

3