Close packing of atoms is simply close packing of spheres. There is only one way to efficiently arrange circles in 2D…and we can visualise the stacking with 2D drawings…
…each sphere (seen from above) is surrounded by 6 other spheres. This is a single layer, the next step is to stack the layers.
The most efficient place to put the next spheres is in the depressions between each sphere on layer one. So, we have one of two options.
We can either have rows in A or rows in B. Due to the size of the spheres it is not possible to fill ALL holes (A and B).
In position A:
In position B:
The fact is that the two sketches above are mirror images. This means it doesn’t matter which holes the second layer fall into! The important thing here is the creation of a new hole…directly above the spheres in layer 1. So for layer 3, we either have a repeat of layer 1 or a new layer in a new position.
Making 3 different layers with a new position:
Matching layer 1:
There are no spheres/atoms in the same position on the first 3 layer spread, but in the second we can see that layer 3 is in the same position as layer 1.
So, we have several possible outcomes, including:
- 1..2..3..1..2..3..1..2..3……. Three different layers, could be cubic close packing (ccp).
- 1..2..1..2..1..2..1..2..1……. Two different layers, could be hexagonal close packing (hcp).
HCP and CCP are the simplest and most common close packing structures. Each atom is surrounded by 12 other atoms – giving both these structures co-ordination numbers of 12.
So while these are close packed, a body centered cubic arrangement would NOT be, as it only has a co-ordination number of 8. This can also be illustrated with packing densities, where HCP and CCP have a density of 74.1% while BCC (body centered cubic, I) has a density of only 68% (although it is still a common metallic structure).
– Structures of Metallic Elements
In 1883 William Barlow (the curator of the science museum) suggested that the metals would each take one of three structures. These were hcp (hexagonal close packing), ccp (cubic close packing) and bcc (body centered cubic structure – which is NOT close packing).
Note that both hcp and ccp follow a face centered structure and as such have the same packing density, of 74.1%.
This image shows the structures of the metals in the periodic table at room temperature & pressure. I have it correct as far as I know, if you see any problems let me know.
Some metals hold different structures at different temperatures and pressures. This is called Polymorphism and can occur in any compound.
Iron (Fe) is one such metal, and at atmospheric pressure a simple change in temperature will change it’s structure:
Above 1809K – Liquid – (No crystal struture)
1809K – 1665K – Delta-Fe – Body Centered Cubic (bcc)
1665K – 1184K – γ-Fe – Face Centered Cubic (fcc)
Under 1184K – α-Fe – Body Centered Cubic (bcc)
Note – Polymorphism of elements is known as allotropy. An example of allotropy is how carbon forms as diamond, graphite, nanotubules etc.
– Interstitial Holes
Close packing leads to two different types of hole between layers. One has a co-ordination number of 4 and the other of 6 (the number of surrounding spheres/atoms).
These are Tetrahedral (Td) with the co-ordination number of 4 and Octahedral with the co-ordination number of 6.
More to come… **UPDATE ME**