Question:

Number of unit cells in 4 g of X (atomic mass = 40) which crystallises in bcc pattern is iNA - Avogadro number)

Updated On: Jul 6, 2022

$0.1 \, N_A$
$2 \times 0.1 \, N_A$
$\frac{0.1 N_A}{2}$
$ 2 \times N_A$

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The Correct Option is C

Solution and Explanation

4g of $X = \frac{4}{40}$ mole = 0.1 mole = $0.1 \times N_A$ atoms .One bcc unit cell has 2 atoms, therefore, no. of unit cells in $0.1 N_A$ atoms $ = \frac{0.1 N_A}{2}$

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Notes on Unit Cells

Unit Cell Chemistry

Concepts Used:

Unit Cells

The smallest portion of a crystal lattice which repeats in different directions to form the entire lattice is known as Unit cell.

The characteristics of a unit cell are:

The dimensions are measured along the three edges, a, b and c. These edges can form different angles, they may be mutually perpendicular or may not.
The angles held by the edges are α (between b and c) β (between a and c) and γ (between a and b).

Therefore, a unit cell is characterised by six parameters such as a, b, c and α, β, γ.

Types of Unit Cell:

Numerous unit cells together make a crystal lattice. Constituent particles like atoms, molecules are also present. Each lattice point is occupied by one such particle.

Primitive Unit Cells: In a primitive unit cell constituent particles are present only on the corner positions of a unit cell.
Centred Unit Cells: A centred unit cell contains one or more constituent particles which are present at positions besides the corners.
1. Body-Centered Unit Cell: Such a unit cell contains one constituent particle (atom, molecule or ion) at its body-centre as well as its every corners.
2. Face Centered Unit Cell: Such a unit cell contains one constituent particle present at the centre of each face, as well as its corners.
3. End-Centred Unit Cells: In such a unit cell, one constituent particle is present at the centre of any two opposite faces, as well as its corners.