[Tedjn]

Hi guys,

I'm doing my chemistry homework, which consists of tediously writing down notes that my chem teacher is just going to give to us in typed format eventually. Such is the burden of school. Anyways, that's not the point  

I'm reading about structures of metallic crystals and Zumdahl's 4th Ed has lots of good colorful pictures that have helped me out more than once, but I'm a little lost regarding the closest packed structures.

First of all, under the picture of an aba arrangement is the statement "the unit cell is the hexagonal prism illustrated here in red." From what I see, what is illustrated is clearly a rectangular prism, so if anyone can explain that to me, that'll be great.

Also, if I just put the first issue I mentioned above to the back of my head, I'm wondering why the hcp structure only has 12 nearest neighbors and not 14.

I get that the central atom is surrounded by 6, but how come there are only three above and three below and three above, not four? In all the pictures, there seems to be a clear space for a fourth sphere that isn't there.

Can anyone explain this to me? Thanks.

[mike]

Also, if I just put the first issue I mentioned above to the back of my head, I'm wondering why the hcp structure only has 12 nearest neighbors and not 14.

I am not sure what your picture looks like, but here is one I drew to show how there can be 12.

[Tedjn]

Ah thanks! The picture in the book was from the side and the spheres looked small (like they could each fit in one dimple, and there's 5 in your picture--but the scale in the book looked like it could fit 4). Your picture made it clear that only three can fit

Although not important, if someone has Zumdahl's 4th Ed. and can check page 456 Figure 10.14 is the one with the hexagonal structure that looks like a rectangular pyramid. If anyone could explain that to me, it'll be great.