Vanderwaals forces and induced dipole forces
I read that Van der Waals forces are induced dipole-dipole forces but I thought they were 2 different things can someone please explain?
Original post by Anonymous
I read that Van der Waals forces are induced dipole-dipole forces but I thought they were 2 different things can someone please explain?
They are one and the same.
A dipole is the term used to describe two zones of opposing charge within a chemical bond. Induced dipoles are dipoles that arise from the movement of electrons in one molecule repelling electrons in another, leading to there being temporary zones of opposite charges in each molecule and therefore causing weak attraction between the two molecules. This weak attraction is what is referred to as the ‘van der Waals’ force or the induced dipole-dipole interaction.
You are probably thinking of permanent dipole-dipole interactions, which are completely different- these arise from charge disparities in covalent bonds as a result of differing electronegativities.
van der Waals forces (strictly speaking, London dispersion forces) are different because they can occur even between neutral atoms or molecules with no permanent dipole. They arise because of an intrisic quantum mechanical fluctuation of charge cloud on one atom/molecule which induces a corresponding dipole on the neighbouring atom/molecule. These dipoles are called transient, because their average values over time are zero. However, at any given time, they are non-zero, and give rise to a weak attractive force which decreases as the reciprocal of sixth power of distance between molecules. Although a similar induced dipole can arise when a molecule with permanent dipole approaches another atom/molecule, the difference is that vdW forces will arise without (or in addition to) the induced dipole arising from permanent dipole. Furthermore, the permanent and corresponding induced dipole are non-transient, i.e. their time-averaged values are non-zero.
Original post by lordaxil
van der Waals forces (strictly speaking, London dispersion forces) are different because they can occur even between neutral atoms or molecules with no permanent dipole. They arise because of an intrisic quantum mechanical fluctuation of charge cloud on one atom/molecule which induces a corresponding dipole on the neighbouring atom/molecule. These dipoles are called transient, because their average values over time are zero. However, at any given time, they are non-zero, and give rise to a weak attractive force which decreases as the reciprocal of sixth power of distance between molecules. Although a similar induced dipole can arise when a molecule with permanent dipole approaches another atom/molecule, the difference is that vdW forces will arise without (or in addition to) the induced dipole arising from permanent dipole. Furthermore, the permanent and corresponding induced dipole are non-transient, i.e. their time-averaged values are non-zero.
We probably should have asked the OP what level they are at, as at A level, that response would not be expected (though it is still correct).
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