A unit cell is (a) The smallest group of atoms which when regularly repealed for

Question

A unit cell is (a) The smallest group of atoms which when regularly repealed forms the crystal (b) A group of atoms which form a cubic arrangement (c) A unit cube containing the smallest number of atoms (d) The smallest group of atoms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

Explanation / Answer

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.

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