BR Example 1.6

[Dochopeful13]

Hi All

I had a quick question about bond angles. The question is asking for the bond angle between C and H in formaldehyde. I thought it would be 120 degrees. However, the answer is 118 because the oxygen has two pairs of electrons that repel the electrons in the carbon hydrogen bond. I thought only lone pairs on the central atom reduced bond angles. Do lone pairs on any atom reduce bond angles? Can someone please clarify this for me?

Thanks!

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[Saigon]

http://www.adichemistry.com/general/chemicalbond/vsepr/vsepr-theory.html

 

[Dochopeful13]

http://www.adichemistry.com/general/chemicalbond/vsepr/vsepr-theory.html

Thank you very much for your help. The carbonyl group decreases the bond angles. Do all double bonds do this Ina molecule? For an Example would a C and C double bond have the same bond angle decreasing effect?

Thanks!

 

[Saigon]

No the lone pairs are key. For example, a molecule with a tetrahedral geometry would have angles slightly less than 109.5 degrees if one of the central atom's ligands had lone pairs. Its not the bonding electrons that reduce the angle it's the greater repulsion of the lone pairs in comparison to the bonding electrons. The molecule is more stable when the lone pairs are farthest apart from one another rather than if the bonding electrons are farthest apart from one another. Lone pairs have greater repulsion because the charge covers a greater area unlike bonding pairs whose charge is confined between two nuclei.

Thank you very much for your help. The carbonyl group decreases the bond angles. Do all double bonds do this Ina molecule? For an Example would a C and C double bond have the same bond angle decreasing effect?

Thanks!