Question:

A metal crystallises in a face centred cubic structure. If the edge length of its unit cell is $'a'$, the closest approach between two atoms in metallic crystal will be :

Updated On: Jun 26, 2024
  • $\sqrt{2} a $
  • $\frac{a}{\sqrt{2}}$
  • $2a$
  • $2\sqrt{2} a$
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The Correct Option is B

Solution and Explanation

In FCC, one of the face is like





By $\Delta ABC$,

$2a^{2} = 16r^{2}$
$\Rightarrow r^{2} = \frac{1}{8} a^{2}$
$\Rightarrow r = \frac{1}{2 \sqrt{2}} a$

Distance of closest approach $ = 2r = \frac{a}{\sqrt{2}}$
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Concepts Used:

Solid State

Solids are substances that are featured by a definite shape, volume, and high density. In the solid-state, the composed particles are arranged in several manners. Solid-state, in simple terms, means "no moving parts." Thus solid-state electronic devices are the ones inclusive of solid components that don’t change their position. Solid is a state of matter where the composed particles are arranged close to each other. The composed particles can be either atoms, molecules, or ions. 

Solid State

Types of Solids:

Based on the nature of the order that is present in the arrangement of their constituent particles solids can be divided into two types;

  • Amorphous solids behave the same as super cool liquids due to the arrangement of constituent particles in short-range order. They are isotropic and have a broad melting point (range is about greater than 5°C).
  • Crystalline solids have a fixed shape and the constituent particles are arranged in a long-range order.