How are Van der Waals interactions formed between molecules?

This questions relies on understanding of electronegativity, instantaneous dipoles and temporary induced dipoles.

Van der Waal's forces (also called London Dispersion Forces) are weak intermolecular forces between molecules.

Electronegativity is the measure of the tendency of an atom to attract electrons, and covalent bonds are formed between two atoms that have similar electronegativity values. When these values are the same in two covalently bonded atoms, both atoms in the molecule exert the same 'pull' on the shared electrons and so the molecule is termed 'non-polar', meaning electrons are distributed equally between the bonded atoms.

However, since electrons are constantly in motion, at any one time there could be more electrons around only one of the bonded atoms, such that an instantaneous dipole is formed. Since electrons are negatively charged, this means one side of the molecule is now more negatively charged than the other as it has more electrons.

After this, since like charges repel like charges (a negative charge will repel another negative charge), this instantaneous dipole will repel the negative electrons in a neighbouring molecule, forming a 'temporary induced dipole' in the neighbouring molecule. This means the neighbouring molecule now has a more positive side around one of its atoms since most of its electrons were pushed to the other side of the molecule. The other side of the neighbouring molecule where all the electrons were repelled to is now more negative than it was before. 

Due to this, the induced positive and instantaneous negative side of the neighbouring and original molecules are now weakly attracted to each other due to positive-negative attraction.

This temporary induced dipole in the neighbouring molecule then repeats the inducing effect on molecules neighbouring it, such that this Van der Waals attraction force is present between many molecules in a substance.

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