1. Feb 23, 2009

jeebs

Hi people, here is my problem:

"One form of boron nitride (BN) has a hexagonal layer structure similar to that of graphite. Assuming an ionic structure as shown below (*see attachment*), with a B to N nearest neighbour distance of 0.15 nm, write down the first three terms in the expression for the Madelung constant."

The attempt at a solution

This is the first time I have came across a problem like this one so I am having a bit of a stab in the dark here.

- I thought it might help to look up the Boron and Nitrogen atoms' proton numbers, which are 5 and 7 respectively.

- I had a look at my notes but they aren't much use. From what I can gather, the Madelung energy is the energy of all the bonds of the nearest neighbouring ions plus the energy of all the bonds of the second nearest ions plus the third nearest, and the next nearest, and the next nearest and so on. However, for this problem i only have to consider the 1st, 2nd and 3rd closest neighbours. This is where I get stuck:

The diagram shows one layer of a hexagonal pattern, and tells me the length of the nearest-neighbour bond. BUT, it also says that it has a graphite-like structure of layers. So, how am I supposed to find out the 2nd and 3rd neighbour distances?

do I just assume a hexagonal close packed 3D structure and just mess around with that to find the 2nd and 3rd distances?
or is there something I'm not considering?

also, what goes on in a boron - nitrogen ionic bond?
am I right in thinking that Boron has 3 outer shell electrons, and Nitrogen has 5, so the ionic bond that forms means Boron's 3 outer electrons are transferred to the Nitrogen to make a 3+ Boron ion and a 3- Nitrogen ion?
I think this may be important for the problem but I am unsure.

Thanks.

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