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Mean position of the negative charges and positive changes are not at the same point (polar molecule) how will that affect another molecule nearby, why it feels a relatively large force? Will the type of another molecule affect this dipole-dipole force, if the other molecule is a water molecule, what if the other molecule is an oxygen molecule?

It is stated in Feynman's lecture vol1.

"There are different cases: in the water molecule, for example, the negative charges sit more on the oxygen, and the mean positions of the negative charges and of the positive charges are not at the same point; consequently, another molecule nearby feels a relatively large force, which is called a dipole-dipole force."

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Since the "mean positions" of the positive and negative charge are different, the magnitude of the force they exert on the other molecule is not the same. Hence they don't cancel out and the other molecule is acted upon by a net non-zero force.

In the water molecule, the electronegativity of the $O$ atom and the $H$ atoms differ, oxygen being more electronegative. Hence the elctron cloud is distorted toward the $O$ atom.

You may also want to see : 1) Electric dipole and 2) Polarizability

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  • $\begingroup$ Thanks, it helps. $\endgroup$ – Vivien dong Jun 25 at 5:07

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