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Crystal Lattice

A regular 3D arrangement of points [constituent particles] in space is called a crystal lattice.

Characteristics of Crystal Lattice:-

  1. Each point in a crystal lattice represents one constituent particle which may be an atom, a molecule or an ion.
  2. Each point in a lattice is called lattice point/lattice site.
  3. Lattice points are joined by straight lines to bring out the geometry of lattice.

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Unit Cells 

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It is the smallest portion of a crystal lattice. when it repeated in different directions generates the entire lattice. [Delhi 2011/CBSE 2011]

A unit cell is characterized by:

(i) Its dimensions along the three edges, aand c which may or may not be mutually perpendicular.

(ii) Angles between the edges, α (between b and c) β (between and c) and γ (between a and b). Thus, a unit cell is characterized by six parameters a, b, c, α, β and γ.

The Solid State Notes

The Solid State

Types of Unit Cell

Primitive Unit Cells

If the constituent particles of a crystal lattice are present only on the corner positions of a unit cell, it is known as primitive unit cell.

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Centered Unit Cells

  • If the constituent particles of a crystal lattice are present at positions other than corners in addition to those at corners, it is known as centered unit cell.
  • Centered unit cells are of three types:
    • Body-Centred Unit Cells:
The Solid State Notes

If the constituent particles of a unit cell are present at its body-centre besides the ones that are at its corners.

  • Face-Centred Unit Cells:
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If the constituent particles of a unit cell are present at the center of each face, besides the ones that are at its corners.

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If the constituent particles of a unit cell are present the center of any two opposite faces besides the ones present at its corners.

There are 7 primitive unit cells and 14 possible 3D lattices [Bravais Lattice]

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Q. Define Primitive Unit Cells? (CBSE 2015/2011/Delhi 2009)

Q 1.10. Give the significance of a lattice point. (NCERT Book)
Ans: The lattice point denotes the position of a particular constituent in the crystal lattice. It may be atom, ion or a molecule. The arrangement of the lattice points in space is responsible for the shape of a particular crystalline solid.

Q. 1.11. Name the parameters that characterise a unit cell. (NCERT Book)
Ans: A unit cell is characterised by the following parameters:
(i)the dimensions of unit cell along three edges: a, b and c.
(ii)the angles between the edges: α (between b and c); β (between a and c) and γ (between a and b)

1.12. Distinguish between :
(i) Hexagonal and monoclinic unit cells
(ii) Face-centred and end-centred unit cells.
Ans:
(i) In a hexagonal unit cell :
a = b # c; α = β = 90° and γ = 120°
In a monoclinic unit cell :
a # b # c and α = γ = 90° and β # 90°
(ii) In a face-centered unit cell, constituent particles are located at all the corners as well as at the centres of all the faces.
In end-centered unit cell, constituent particles are located at all the corners as well as at the centres of two opposite faces. (C.B.S.E Foreign 2015)

The Solid State NCERT Solution Class 12th Chemistry
The Solid State NCERT Solution Class 12th Chemistry

Q. Distinguish between Unit Cell & Crystal Lattice?

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