Where does the giant ionic structure end?

[weeeman]

I was just revising and came across the giant ionic structure, the cube. Well i was thinking, the face of the cube would have charges on it wouldn't it?

For example NaCl. The crystal coordination number is 6:6 so 6 Cl's should surround a Na atom and visa versa. Well, at the face of the structure there can not be 6 Cl's surrounding an Na atom and then 6 Na's surrounding those Cl atoms. It would be endless right? The Cl in the centre of all faces only has 5 Na atoms surrounding it.

The Na in the centre edge has 4 Cl's surrounding it.

Why is this?

[CaptainPanic]

Why is this?

Good question... I've always struggled with this too

 

Well, you already gave the obvious answer yourself. Why is this? Because otherwise the crystal would have to go on forever. It is very impractical to have an infinitely sized NaCl crystal on your food (NaCl = tablesalt).

 

So, obviously, there are imperfections to the crystal lattice, at the edges (but often also in the middle of the crystal!). With an "imperfection", I mean any place where either a Na or a Cl is not surrounded by the perfect 6 ions of the other type. I think that these imperfections will make the crystal a little weaker, but overall the attractions between the ions are so strong that the rest of the bonds keep the entire crystal together.

 

And actually, the imperfections at the edge will be "felt" by the ions several layers deep into the crystal. But even in a small crystal (let's say a grain of salt is 0.05 mg), there are about 515,317,473,900,394,000 ions of both Na and Cl! So, a few imperfections on the surface are not going to ruin the rest of the crystal.

 

Personally, I am even more amazed that diamond is stable. If you look at the edge of a diamond, my first instinct would be that it should have some radicals (carbon atoms with only 2 or 3 bonds, instead of the perfect 4).

[weeeman]

Good question... I've always struggled with this too

 

Well, you already gave the obvious answer yourself. Why is this? Because otherwise the crystal would have to go on forever. It is very impractical to have an infinitely sized NaCl crystal on your food (NaCl = tablesalt).

 

So, obviously, there are imperfections to the crystal lattice, at the edges (but often also in the middle of the crystal!). With an "imperfection", I mean any place where either a Na or a Cl is not surrounded by the perfect 6 ions of the other type. I think that these imperfections will make the crystal a little weaker, but overall the attractions between the ions are so strong that the rest of the bonds keep the entire crystal together.

 

And actually, the imperfections at the edge will be "felt" by the ions several layers deep into the crystal. But even in a small crystal (let's say a grain of salt is 0.05 mg), there are about 515,317,473,900,394,000 ions of both Na and Cl! So, a few imperfections on the surface are not going to ruin the rest of the crystal.

 

Personally, I am even more amazed that diamond is stable. If you look at the edge of a diamond, my first instinct would be that it should have some radicals (carbon atoms with only 2 or 3 bonds, instead of the perfect 4).

 

So what you are saying is that the number of atoms with imperfections on the surface are basically cancelled out by the sheer number of perfect atoms within the crystal and therefor the imperfections are negligible. This makes perfect sense to me and is made all the more understandable by your example.

 

Yes that was another of the structure i was thinking of. I think the greatest problem would be at the points of the structure where there is only 1 bond.

Surely though, the same principals would apply, the number of carbons withing the structure and so electron density is greater within the structure, meaning that the free electrons are less likely to have an effect?

[John Cuthber]

The surfaces of crystals react with air, water or whatever else is around. So, for example, that diamond would lose the single C from it's tip and acquire an OH group.

[weeeman]

so the tips of diamond are an alcohol?

 

This brought to my mind aluminum. Aluminium forms a layer of Al oxide so your idea makes sense to me.

[Suxamethonium]

Yeah, I learnt in high school that it was varying amounts of OH and H terminations at the end carbons in the structure (and that the exact ratio determines the hydrophobicity of the diamond). But I don't really know how accurate that model is- being high school, was probably over-simplified like everything else.

 

Likewise, with a salt. If you put it in a liquid, the 'solvent' would take the place of the missing ions (and in some cases this would initiate solvation). In air, this principal may also occur in the 'hydrates' with water molecules also shielding the edges of the latice- but this is just speculation on my part.

[weeeman]

It seems high school may have simplified their teaching but you have learnt interesting things.

 

Thanks for all the information guys