Chapter 4: Unit 12. Resonance Structures

Resonance structure

Resonance structures are multiple Lewis dot structures having the same arrangement of atoms but a different arrangement of electrons. This is possible in case of multiple valid Lewis dot structures.The hybrid is a composite of all resonance structures that spreads out electron pairs in multiple bonds and lone pairs. In resonating structures, electron pairs move from one part of the atom to the other part. Resonating structures are shown by a double headed arrow.

All contributing structures must:

1. Have the same number of valence electrons.
2. Obey the rules of covalent bonding.
   • No more than 2 electrons in the valence shell of H.
   • No more than 8 electrons in the valence shell of a 2nd period element.
   • 3rd period elements, such as P and S, may have up to 12 electrons in their valence shells.

3. Differ only in distribution of valence electrons; the position of all nuclei must be the same.
4. Have the same number of paired and unpaired electrons.

Curved arrow:   A symbol used to show the redistribution of valence electrons:

• the tail of the arrow identifies a pair of electrons whose location is changing.
• the head of the arrow identifies the the new location of the involved pair of electrons.

In using curved arrows, there are only two allowed types of electron redistribution:

• from a bond to an adjacent atom.
• from an atom to an adjacent bond

Here is another example of resonance.

Questions:

1. Draw resonance structures of NO₂-, C₆H_{6, CH3COO-, HCO3-}