A unit cell is a The smallest group of atoms which when regu

A unit cell is (a) The smallest group of atoms which when regularly repealed forms the crystal (b) A group of atoms which form a cubic arrangement (c) A unit cube containing the smallest number of atoms (d) The smallest group of atoms which will diffract X-rays. Crystalline materials atoms: (a) are pack in periodic (b) have no periodic packing (c) tend to be densely packed (d) a, c ATOMIC PACKING FACTOR (Volume of atoms in unit cell/Volume of unit cell) tor HCP crystal structure (a) 0.52 (b) 0.68 (c) 0.74 (d) 1 Single crystal is Anisotropic: (a) True (b) False (c) Depends on type of unit cell (d) None Polycrystalline material are anisotropic: (a) True (b) False (c) If grains are textured (d) If grains are randomly oriented

Solution

Since the exact question number is not specified, let us go ahead and address the question number 8:

The option (a) is the right answer.

A unit cell can be defined as a basic building block formed by a group of atoms, which when repeated across all the three dimensions in space, can regenerate the whole crystal structure. The unit cell shape can be a cubic structure as well as other structures - such as tetragonal, Rhombohedral, hexagonal etc. Cubic arrangement is just one of the possibilities, but not a necessary requirement - hence the options (b) and (c) are ruled out.

Lastly the X-ray diffraction method just helps us to determine the distance between planes of atoms in a crystal; this is described in Bragg\'s equation. Any crystal surface diffracts the X-ray, in various patterns. But that does not define what a unit cell is; hence the option (d) is also ruled out.