The Periodic Table

London dispersion forces (LDF, also known as dispersion forces, London forces, instantaneous dipole–induced dipole forces, or loosely van der Waals forces) are a type of force acting between atoms and molecules. They are part of the van der Waals forces. The LDF is named after the German-American physicist Fritz London. The LDF is a weak intermolecular force arising from quantum-induced instantaneous polarization multipoles in molecules. They can therefore act between molecules without permanent multipole moments. London forces are exhibited by nonpolar molecules because of the correlated movements...

IͶΔ

11:49 AM

Polar molecules have an inherent +/- separation. So the electrostatic forces cause most of the intermoelcular forces as much as I know.

Oh wait.

I see where your problem is @Cop.

Well, no.

CopperKettle

> For most molecules, including those which can hydrogen bond, dispersion forces contribute the most to the overall intermolecular bonding. Describing dispersion forces as the weakest kind of intermolecular force is wrong.

(Chemguide)

IͶΔ

@Cop in H2, for one very small moment, you get 50.0001 and 49.9999 percent of electron charge density around the atoms instead of 50/50.

Because they move. Even though that's really really fast.

CopperKettle

I read about it. (0:

BBL!

IͶΔ

In something like H2O, let's think that for example, if the normal routine is 85% electron cloud density around O, and 15% around H, because the electrons move, it's gonna be 84.9% and 15.1%.

So polarity isn't anything being locked.

It's just more electron cloud density on one atom.

@CopperKettle Whoa, let's not forget something.

That dispersion forces usually can't overcome polar attractions doesn't mean they can't be strong.

They can be strong, but they're the weak ones when we're talking about tiny molecules.

That's my point.

pH13

12:25 PM

@IͶΔ that's wrong, it's not about time but about probabilities

IͶΔ

@pH13 They always use time when they wanna (over-)simplify probabilities.

pH13

that doesn't change that it is wrong :P

IͶΔ

Well, gimme an oversimplification that can't be wrong.

Apples produce gasses. Bubbles are filled with gas. -> Apples are inside bubbles @pH13.

We burn gasses. Hence we burn bubbles.

pH13

more cheese -> more holes in cheese
more holes in cheese -> less cheese
---
more cheese -> less cheese

IͶΔ

Yes. It works.

Nice! $%this answer is great!$ — IͶΔ Dec 28 '15 at 10:31

Oh. I used Mathjax.

CopperKettle

2:17 PM

> When an electron is added to a neutral atom (i.e., first electron affinity) energy is released

How can this be? It acquires an atom, surely it must consume some energy in the process, not release some energy into the medium.

I mean, it must take us some energy to push an electron where it does not belong.

A neutral atom won't just gulp an electron on its own will, and then even release some energy. O_o

getafix

2:35 PM

@IͶΔ Thanks. Happy to help. will continue to do so every now and then.

getafix

2:45 PM

@CopperKettle: Eea = (Einitial − Efinal)= −ΔE(to attach an electron) That's how it's usually defined.

This your attachment process.. X + e− → X− + energy

alternatively, if you look at X− → X + e− , and if you are convinced that ionisation requires an input of energy(positive value), then you can convince yourself why Ea must have a negative value..

Or think of it from an electrostatics point of view, setting the potential at infinity = 0, you move an electron closer towards a nucleus.. what would be the sign of the work done, and consequently what would be the sign of the change in energy for the process.

And just to point out, its not negative for all species..bear in mind what's more "stable" you reactant or your product.. i.e species X or the ion X-

CopperKettle

4:11 PM

@getafix - thank you!

Although I understood nothing.

(0:

I don't understand why is there a release of energy when, say, O(0) turns into O(-).

An electron has been added. The atom will radiate energy to celebrate the event?

Where is the energy coming from?

From ChemWiki:

> When an electron is added to a neutral atom (i.e., first electron affinity) energy is released; thus, the first electron affinities are negative.

Released from what?

The atom was stable, it is now an unstable ion.

I found one answer that at least discusses the issue of where this energy comes from

I'm amazed at how only this one guy actually addressed the issue there, and the others only repeated the formula in a circular logic sort of way: "it is released because there's that minus sign there".

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