How can I identify dipoles in molecules?

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Answer 1 · 2015-06-04T21:52:55

You look for electronegativity differences between the atoms that are bonded to each other.

A bond dipole depends on electronegativity differences ( $Δ"EN"$ ) between the atoms in the bond.

This causes the electrons in the bond to spend more time around one atom than the other.

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For example, in $"HCl"$ , the $"Cl"$ is more electronegative than $"H"$ , so the electrons spend more time around $"Cl"$ than $"H"$ .

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We indicate a bond dipole by an arrow with a "+" at one end and pointing towards the negative end of the bond.

If $Δ"EN" < 0.5$ , we usually say that the bond is ionic.

EXAMPLE

Identify the bond dipoles on chloromethane, $"CH"_3"C"l$ .

Solution

The structure of chloromethane is

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$Δ"EN"$ for the $"C-H"$ bond is 0.4. The $"C-H"$ bond is nonpolar.

$Δ"EN"$ for the $"C-Cl"$ bond is 0.6. The $"C-Cl"$ bond is polar.

The $"Cl"$ atom is the negative end of the bond, so there is a $"C-Cl"$ bond dipole.

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