In case of hcp structure, how are spheres in first, second and third layers arranged?
Answers
Answer:
The packing of spheres can describe the solid structures of crystals. In a crystal structure, the centers of atoms, ions, or molecules lie on the lattice points. Atoms are assumed to be spherical to explain the bonding and structures of metallic crystals. These spherical particles can be packed into different arrangements. In closest packed structures, the arrangement of the spheres are densely packed in order to take up the greatest amount of space possible.
Types of Holes From Close-Packing of Spheres
When a single layer of spheres is arranged into the shape of a hexagon, gaps are left uncovered. The hole formed between three spheres is called a trigonal hole because it resembles a triangle. In the example below, two out of the the six trigonal holes have been highlighted green.
File:Physical_Chemistry/Physical_Properties_of_Matter/Solids/Crystal_Lattice/Closest_Pack_Structures/trigonal_holes_jpeg.JPG
Once the first layer of spheres is laid down, a second layer may be placed on top of it. The second layer of spheres may be placed to cover the trigonal holes from the first layer. Holes now exist between the first layer (the orange spheres) and the second (the lime spheres), but this time the holes are different. The triangular-shaped hole created over a orange sphere from the first layer is known as a tetrahedral hole. A hole from the second layer that also falls directly over a hole in the first layer is called an octahedral hole.
Closest Pack Crystal Structures
Hexagonal Closest Packed (HCP)
In a hexagonal closest packed structure, the third layer has the same arrangement of spheres as the first layer and covers all the tetrahedral holes. Since the structure repeats itself after every two layers, the stacking for hcp may be described as "a-b-a-b-a-b." The atoms in a hexagonal closest packed structure efficiently occupy 74% of space while 26% is empty space.
File:Physical_Chemistry/Physical_Properties_of_Matter/Solids/Crystal_Lattice/Closest_Pack_Structures/hcp2_jped.JPG
Cubic Closest Packed (CCP)
The arrangement in a cubic closest packing also efficiently fills up 74% of space. Similar to hexagonal closest packing, the second layer of spheres is placed on to of half of the depressions of the first layer. The third layer is completely different than that first two layers and is stacked in the depressions of the second layer, thus covering all of the octahedral holes. The spheres in the third layer are not in line with those in layer A, and the structure does not repeat until a fourth layer is added. The fourth layer is the same as the first layer, so the arrangement of layers is "a-b-c-a-b-c."
File:Physical_Chemistry/Physical_Properties_of_Matter/Solids/Crystal_Lattice/Closest_Pack_Structures/ccp_jpeg.jpg
Coordination Number and Number of Atoms Per Unit Cell
A unit cell is the smallest representation of an entire crystal. All crystal lattices are built of repeating unit cells. In a unit cell, an atom's coordination number is the number of atoms it is touching.
The hexagonal closest packed (hcp) has a coordination number of 12 and contains 6 atoms per unit cell.
The face-centered cubic (fcc) has a coordination number of 12 and contains 4 atoms per unit cell.
The body-centered cubic (bcc) has a coordination number of 8 and contains 2 atoms per unit cell.
The simple cubic has a coordination number of 6 and contains 1 atom per unit cell.
Simple Unit Cell
Answer:
in the first method , each successive layer of spheres covers gaps in previous layer. three neighboring spheres in first layer will form a hollow space where they meet. spheres in one layer align to fit in hollows formed in the previous layer. the third layer aligns directly above the first layer.