S2.Induced dipole– Induced dipole interactions.
Vander Waals bond , London dispersion force and induced dipole – induced dipole interactions all mean the same. This type of interaction is operative in non – polar molecules and it is the weakest type of attraction.
We know that , iodine exist in solid state at room temperature.As it is a solid it is understood that the iodine molecules are held together by some force. Iodine is a non polar molecule and thus it does not have a dipole in it .So the question is , what binds one iodine molecule to other?
In a non polar species, at any given moment in time, temporary dipoles can be created owing to the position of electrons. The electrons are moving in the respective orbitals.There is a chance that, at a given instant, electrons aren’t symmetrically distributed around the nucleus.Thus, a partial negative charge(-δ) is developed at the end where there are more electrons .Thus, if there are more electrons on the right side, the right side will develop partial negative charge.Naturally, the other end becomes partial positive end(+δ).This positive end, will now attract the electron density of the adjacent species, towards itself.And so a dipole will be induced in the adjacent species as well. Thus, a dipole in one induces a dipole in other species .This continues and it accounts for the induced dipole – induced dipole interactions. The bond thus created is called Vander Waals bond and the force that binds to-gether the atoms/molecules is called London dispersion force(as it was developed by Fritz London in 1930).
Polarizability is the ability of a species to form instantaneous dipoles i.e it is measure of how easily the electron cloud can be distorted.
Larger the size of the atom , more loosely are the valence electrons held.This is because they encounter less nuclear force and they are away from the nucleus. Thus, it becomes easy to distort them.So, polarizability of bigger atoms/ions is more than smaller ones.Thus, the Vander waals bonds have a more pronounced effect in larger atoms/ ions/ molecules.As the
size of the species gets bigger→large α →the Vander Waal bond gets stronger→ →increase in melting and boiling points of these species.
Q: Which of the two has a greater boiling point – Argon and Helium ?
A: Argon is bigger than Helium.
In the next post we start talking about the most important secondary bond. Till then, be a perpetual student of life and keep learning…
Good Day !
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