a) attraction between ions causes the dispersion forces.
Attraction between ions is a temporary attractive force which occurs when electrons in two consecutive adjacently placed atoms occupies positional confined areas that make the atoms acting like Temporary dipoles.
Dispersion forces or London Dispersion forces is the weakest force caused due to motion of electron.
On increasing the number of electrons the magnitude to Dispersion forces increases.
Dispersion forces also depends upon the atomic or the molecular weight of the given substance.
It is caused by an unequal and uneven distribution of the electrons inside an atom.
This causes a slight positive and slight negative charge to be formed inside an atom by establishing a temporary dipole. This temporary dipole induces a nearby atom by inducing another temporary dipole in the other one.
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