You are Here: Home

Announcements Posted on
Talking about ISA/EMPA specifics is against our guidelines - read more here 28-04-2016
1. Okay, so the lattice of solid NaCl has the form of a cube, and thus when you start summing the interactions you get where r_0 is the distance Na-Cl from the central Na+ ion to the six closest Cl- ions.

The second term in the expansion is third fourth fifth and I could calculate some more, but they get more and more geometrically tedious.

I know that it converges to 1.74... but want to firstly set up the general term for the series, and then prove that it converges (comparison test or similar depending on how the series looks). I've had a look at the wiki article but it didn't seem to help me much, or at least not in a way I understood.

edit: I've seen the series they give on wikipedia, () but don't directly see where this comes from. And why are they summing from -infty to +infty? That would mean we are looking at an infinitely large structure of ions? Also, wouldn't it be equally possible to just sum from 0 to infinity and multiply the result by 2, because cubes are symmetric...

But apparently the message to take home is that the 'normal' way that we use to 'show' what the series should converge to, the first few terms of which you've written above, doesn't actually converge! (I definitely remember writing 'it can be shown that this series converges' in an exam ... oh, the beauty of proof by omission!)
So this is definitely interesting, and I'll look into it more properly in the summer. Who knew that basically *all* the textbooks on this blatantly lie?
3. I'll PM you my email.

Basically, I was revising for something unrelated, but my book ("Why chemical reactions happen") mentioned this a bit, and I looked at it because I found it interesting.

edit: Thanks a lot for taking your time to look at this, I'm really grateful! And yes, proof by omission is a good one Especially when you can't remember the proof.

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: April 9, 2008
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How to predict exam questions

No crystal ball required