London Forces

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#1
London forces:
If the fluctuations of electron densities as electrons orbit within a molecule are random, why does it induce a dipole (in a neighbouring molecule) which attracts them together rather than repelling them apart?
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5 years ago
#2
(Original post by dcstudent999)
London forces:
If the fluctuations of electron densities as electrons orbit within a molecule are random, why does it induce a dipole (in a neighbouring molecule) which attracts them together rather than repelling them apart?
The electrons are orbiting and just generally doing their thing in one atom (atom 1), but when they get close to a neighbouring atom (atom 2), they repel the electrons in atom 2 away from the side that atom 1 is on.

(for the purposes of this diagram apostrophe, comma and hyphen are electrons)
atom 1 atom 2
-O ,'- -O ,'- <---these electrons are repelled away from atom 1

As a result of the repulsion, there is a higher number of electrons (higher electron density) on the side of atom 2 further away from atom 1.

Now, negative charge is just when there is a high electron density, whilst positive charge is when there is a low electron density.

Because of the high number of electrons on the right side of atom 1, the right side of atom one has a partially negative charge, whilst the left side of atom 1 has fewer electrons and therefore a partially positive charge.

The same rules apply for atom 2. As a result the right side of atom 1 has a partially negative charge, and the left side of atom 2 has a partially positive charge.

And the rest is basic physics - opposite charges attract, and there's your London force!
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#3
(Original post by r_98)
The electrons are orbiting and just generally doing their thing in one atom (atom 1), but when they get close to a neighbouring atom (atom 2), they repel the electrons in atom 2 away from the side that atom 1 is on.

(for the purposes of this diagram apostrophe, comma and hyphen are electrons)
atom 1 atom 2
-O ,'- -O ,'- <---these electrons are repelled away from atom 1

As a result of the repulsion, there is a higher number of electrons (higher electron density) on the side of atom 2 further away from atom 1.

Now, negative charge is just when there is a high electron density, whilst positive charge is when there is a low electron density.

Because of the high number of electrons on the right side of atom 1, the right side of atom one has a partially negative charge, whilst the left side of atom 1 has fewer electrons and therefore a partially positive charge.

The same rules apply for atom 2. As a result the right side of atom 1 has a partially negative charge, and the left side of atom 2 has a partially positive charge.

And the rest is basic physics - opposite charges attract, and there's your London force!
Thanks that has explained it quite well. How would you describe and explain a fluctuation in electron density?
0
5 years ago
#4
(Original post by dcstudent999)
Thanks that has explained it quite well. How would you describe and explain a fluctuation in electron density?
The fluctuations in electron density are literally just because of the random movement of electrons in their orbitals
0
5 years ago
#5
(Original post by dcstudent999)
Thanks that has explained it quite well. How would you describe and explain a fluctuation in electron density?
Electrons in an atom are not static, they are constantly moving randomly around the atom and so at any one time, there are going to be more electrons on one side of the atom and so there is a higher electron density on one side of the atom than the other. This creates a temporary dipole. Then, a split second later, the electrons have moved and now there might be more electrons (hence higher e- density) on one side of the atom compared to the other. It is these temporary dipoles that creates weak electrostatic (London-dispersion) forces. The negative side of the atom (where there is higher e- density due to random movements of electrons) will be attracted to the positive side of another atom (where there is lower e- density). In fact, the temporary dipole induces a temporary dipole in adjacent atoms. The higher e- density on one side of an atom will have a δ- charge and so will repel electrons in adjacent atoms, which will induce a δ+ charge on the adjacent side of adjacent atoms (since there is now higher e- density since the electrons are repelled).

Hopefully that helps.
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