Why does a more electronegative heteroatom (i.e. $\ce{Cl}$ or $\ce{O}$) increase the stretching frequency of a carbonyl?

I'd suspect it's that they donate their lone pairs and we end up with a negative charge on the oxygen in one of our resonance structures which makes the electrostatic attraction between the carbon and oxygen larger?

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    $\begingroup$ Also, inductive effects create a larger dipole in the c=o bond which increases the electrostatic contribution to the bond strength $\endgroup$ – RobChem Apr 21 '15 at 11:07

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