ANS:B - body centred cubic (bcc)

In materials science and metallurgy, the term "body-centred cubic" (bcc) refers to a specific type of crystal lattice structure found in certain metallic materials, including some steels. In a body-centred cubic (bcc) structure:

1. Atom Arrangement: Each atom is positioned at the corners of a cube, forming the corners of the unit cell. Additionally, there is one atom located at the centre of the cube, which is why it is termed "body-centred." This central atom is not shared with other unit cells; it belongs exclusively to the central cube.
2. Coordination Number: Each atom in a bcc structure is in contact with eight neighbouring atoms. This coordination number (number of nearest neighbours) is less than the coordination number in face-centred cubic (fcc) structures.
3. Unit Cell: The basic repeating unit of the bcc structure is a cube with atoms at each corner and one atom in the centre.
4. Packing Efficiency: The packing efficiency of a bcc structure is approximately 68%, meaning that approximately 68% of the available space within the unit cell is occupied by atoms.
5. Examples: Some examples of metals that commonly exhibit a bcc crystal structure include alpha iron at temperatures below 912°C, chromium, and certain alloys such as some types of steel.

The body-centred cubic structure is one of the three primary types of crystal lattice structures found in metallic materials, alongside face-centred cubic (fcc) and hexagonal close-packed (hcp) structures. Each structure has distinct properties that influence the mechanical, thermal, and electrical behaviour of the material.