chemistry 12th class cbse notes

Crystal Lattices:

A crystalline solid consist of a large number of small units, called crystals, each of which possesses a definite geometric shape bounded by plane faces. The crystals of a given substance produced under a definite set of conditions are always of the same shape.

Unit Cells:

The smallest part of the crystal lattice is known as a unit cell. Look at the role of the unit cell in the crystal lattice.

Unit Cell In Solid
Unit Cell In Solid

Bravais Lattices:

Bravais (1848) showed from geometrical considerations that there are only seven shapes in which unit cells can exist.

  1. Cubic
  2. Orthorhombic
  3. Rhombohedral
  4. Hexagonal
  5. Tetragonal
  6. Monoclinic
  7. Triclinic.

Seven primitive unit cells in crystals

Seven Primitive Unit Cells and their Possible Variations as Centred Unit Cells
Seven Primitive Unit Cells and their Possible Variations as Centred Unit Cells

Moreover, he also showed that there are basically four types of unit cells depending on the manner in which they are arranged in a given shape.

  1. Primitive
  2. Body-Centered
  3. Face Centered
  4. End Centered.

He also went on to postulate that out of the possible twenty-eight unit cells (i.e. seven shapes ´ four types in each shape = 28 possible unit cells), only fourteen actually would exist. These he postulated based only on symmetry considerations. These fourteen unit cells that actually exist are called Bravais Lattices.

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