1. Resonance Structures

2. Which one of these three structures is the correct one? As it turns out, laboratory measurements show that all three bonds are equal. When more than one Lewis structure can be drawn, the molecule or ion is said to have resonance.

3. Resonance Structure Resonance structures are simply alternate Lewis structures for a given ion or molecule. The structures are changing so quickly that all we see is an average blur instead of being able to detect individual structures.

4. Resonance Structure We draw three Lewis structures because a single structure is insufficient to describe the real structure of the nitrate ion.

5. Resonance Structure ozone as a Lewis structure would be: We draw two Lewis structures because a single structure is insufficient to describe the real structure of ozone.

6. Bond Strength – Bond Length  Thus, triple bonds are stronger and shorter than double and single bonds. Bond# of electrons Bond Order Bond Strength Bond Length Single21WeakestLongest Double42 Triple63StrongestShortest

7. Bond Strength – Bond Length Laboratory analysis show all three of the bonds in the nitrate ion to be the same strength and the same length. They are longer and stronger than single bonds but shorter and weaker than double bonds. What is the bond length between the N-O?

8. Bond Strength – Bond Length The ion behaves as if it were a blend of the three resonance structures. The actual bond length for each N-O bond is 1 and 1/3, because the double bond is shared equally between all the N-O bonds.

9. Polarity of Molecules A polar bond has one end with a positive charge and the other with a negative chare. –The charge difference is due to the difference in electronegativity of the two bonded atoms. The direction of greatest electronegativity is shown with an arrow called a dipole. C---F and C---H

10. Polarity of Molecules Non-polar molecules – the dipoles are all equal and opposite. They cancel out and the molecule is non-polar. Cl---Be---Cl Polar molecules – the dipoles are not equal and opposite so the molecule has a net dipole and is polar. H---C---N Net Dipole

11. Examples Draw all resonance structures, give the bond order and determine the polarity of each ion. 1.NO 2 - 2.ClO 4 - 3.SO 3 4.C 2 H 4