What is the coordination number in BCC

Difference between BCC and FCC

Key difference - BCC vs. FCC

The terms BCC and FCC are used to denote two different arrangements of crystalline structures. BCC stands for body-centered cubic structure while FCC stands for face-centered cubic structure. These are shapes of cubic lattices. Therefore, these arrangements have spheres (atoms, molecules, or ions that make up the lattice) arranged in cubic structures. BCC's unit cell has balls in the corners of a cube and a ball in the center of the cube. Since there are eight corners in a cube, the total number of balls in a BCC unit cell is 9. The FCC unit cell has balls in each corner of a cube and also in the center of each cubic face. Then the unit cell of FCC has 12 balls. The Main difference between BCC and FCC that is The coordination number of BCC is 8 while the coordination number of FCC is 12.

1. Overview and main difference
2. What is BCC?
3. What is FCC?
4. Similarities Between BCC and FCC
5. Side by Side Comparison - BCC vs. FCC in Tabular Form
6. Summary

What is BCC?

The term BCC stands for the body-centered cubic arrangement of spheres (atoms, molecules or ions that make up the lattice). In this arrangement, the balls are in each corner of a cube and one ball is in the center of the cube. A unit cell of a lattice is the smallest unit that resembles the entire structure of the lattice. Since a cube has 8 corners, there are a total of 9 balls in a BCC structure (eight in the corners plus in the middle).

However, each ball in the corner of the unit cell of BCC is a member of the neighboring unit cell. This is because the grid is made up of many unit cells packed together. Since one unit cell contains 8 spheres that are the corners of other unit cells, the coordination number of the BCC structure is known to be eight. Then if you look at the net sum of the balls in a BCC unit cell, it has 2 balls because one corner has 1/8th of bullets. The eight corners together form a sphere, and there is one sphere in the middle, resulting in two spheres.

The balls in the BCC arrangement are not packed tightly. That is, the packing of spheres in BCC is not a close packing as in FCC (face centered cubic) or HCP (hexagonal close packing). The packing factor of BCC is 0.68. The packing factor is the volume of the spheres per volume of the unit cell. Examples of metals with the BCC structure include lithium (Li), sodium (Na), potassium (K), chromium (Cr), and barium (Ba).

What is FCC?

The term FCC stands for the face-centered cubic arrangement of spheres. In this arrangement, the balls are in each corner of a cube (unit cell) and in the centers of each cubic face. Here, too, every ball in the corners is a member of the neighboring unit cell. Apart from that, each sphere in the middle of the cubic face is shared with the neighboring unit cell.

The coordination number of FCC is 12. This is because there are 12 spheres per unit cell that are shared with other unit cells. The net sum of the balls present in the FCC unit cell is 4. It can be calculated as follows.

Total balls in corners = (1/8) x 8 = 1

Total spheres in cubic areas = (1/2) x 6 = 3

Then the total number of balls per unit cell = 1 + 3 = 4

The FCC structure has more packings of balls than that of BCC (the balls pack closer together). The packing factor of the FCC structure is 0.74. This means that the ratio between the volume occupied by spheres and the total volume of the unit cell is 0.74. Some examples of metals with FCC structure are aluminum (Al), copper (Cu), gold (Au), lead (Pb) and nickel (Ni).

What are the Similarities Between BCC and FCC?

  • Both BCC and FCC are forms of arrangement of cubic lattices.
  • The unit cell of both the BCC and FCC structures is a cube.

What is the difference between BCC and FCC?

BCC versus FCC

The term BCC stands for the body-centered cubic arrangement of spheres (atoms, molecules or ions that make up the lattice).The term FCC stands for the face-centered cubic arrangement of spheres.
Arrangement of the balls
BCC has balls in the eight corners of a cube and one ball in the center of the cube.FCC has balls in the eight corners of a cube and also in the middle of the cubic faces.
Coordination number
The coordination number of the BCC structure is 8.The FCC structure coordination number is 12.
Packing factor
The packing factor of BCC is 0.68The FCC packing factor is 0.74
Number of balls in a unit cell
A unit cell from BCC has a total of 2 spheres.A unit cell from FCC has a total of 4 spheres.
Examples
Some examples of metals with the BCC structure include lithium (Li), sodium (Na), potassium (K), chromium (Cr), and barium (Ba).Some examples of metals with FCC structure are aluminum (Al), copper (Cu), gold (Au), lead (Pb) and nickel (Ni).

Summary -BCC vs. FCC

BCC stands for the body-centered cubic arrangement. FCC stands for the face-centered cubic arrangement. These arrangements are used to describe the position of atoms, molecules or ions and the empty spaces present in a lattice structure. The difference between BCC and FCC is that the coordination number of BCC is 8 while the coordination number of FCC is 12.