Structure of metals | Physical Chemistry

Structure of metals | Physical Chemistry

An assembly of positively charged spheres of identical radii which are packed together to fill the space as completely as possible is called a metal.

There are two types of close packings.

To understand the closed packing of atoms in metal structures,

suppose that metal atoms are like hard spherical balls. Take twelve spherical balls and pack them in a box as shown in the figure. The spaces for packaging are larger.

When the box is shaken, the balls rearrange as shown in the figure. The arrangement of these balls is now stable and tighter. It is natural that the balls have a firm arrangement of eleven balls after being stirred.

To understand how different unit cells of the crystal structure develop, consider three balls that lie in the same plane. The fourth ball is inserted into the space created by the other three as the second layer. In this way we get a figure tetrahedral structure. In fact, the fourth ball of the second layer is placed in the Depression created by the three balls of the first layer. These depressions are also called interstices, crevices or voids.

Consider the figure in which eleven balls are present in the first layer (circle with shadow). The balls of the second layer (circle without shadow) can fit into the depressions or interstices created by the first layer. When the balls of the second layer are arranged, all the depressions of the first layer are not occupied. There are two types of depression as ‘a’

(I) The depressions marked ‘b’ are not occupied by the second layer and one can see the ground from looking at the top through depressions ‘b’.

(ii) The new depressions marked ‘a’ are created by the second layer. Through the depressions ‘a’ we cannot see the ground, but balls of the first layer.

Now arrange the balls of third layer in the depression of second layer. When the balls of the third layer are placed above the second layer, then there are two possibilities. Third layer balls may be accommodated in ‘a’- type or ‘lac type interstices or depressions.

  • Cubic close packing
  • Hexagonal close packing

(I) Cubic close packing (ABC, ABC or123,123)

When the atoms of the third layer fit into the interstices marked b, then the atoms of the third layer will not lie directly above

those of the atoms of first layer. This pattern of arrangement is called ABC ABC………… Or

123 123……….. It is named as face centered cubic arrangement. The balls of fourth, seventh
and tenth layers will be in front of each other.

(ii) Hexagonal close packing (AB, AB or 12,12)

When the atoms of the third layer are arranged in such a way that they occupy the depressions created by the second layer. Then these atoms will directly lie above the atoms of first layer.

It is written as ABAB……… or 1212. The balls of third, fifth and seventh layers will be in front

of each other.

Lattice/Space Lattice/Crystal Lattice:

A particular three dimensional arrangement of particles in a crystal is called lattice/space lattice or crystal lattice.

Lattice Point/Lattice Sites:

The space or position occupied by the particles (atoms, ions or molecules) in the crystalline solid is called lattice point or lattice site”.

UNIT CELL

The smallest part of a crystal that describes the characteristics of the entire crystal is called unit cell.

The smallest unit or volume of crystal, which when repeated in three dimensions generates an entire crystal is known as unit cell.

Significance:

  • It is the smallest block/geometrical figure of a crystal.
  • The entire crystal can be built up by repeating it in three dimensions.
  • It shows the structural properties of a crystal.

solid is called lattice point or lattice site”. UNIT CELL

The smallest part of a crystal that describes the characteristics of the entire crystal is called unit cell.

The smallest unit or volume of crystal, which when repeated in three dimensions generates an entire crystal is known as unit cell.

Significance:

  • It is the smallest block/geometrical figure of a crystal.
  • The entire crystal can be built up by repeating it in three dimensions.
  • It shows the structural properties of a crystal.
  • The arrangement of atoms in a unit cell describes their arrangement in whole crystal.
  • The quantitative aspects of a crystal lattice are deduced from the size and shape of its unit cell.

The angle ‘a’ is between the lengths ‘b’ and ‘c’, the angle ‘b’ is between the sides ‘a’ and ‘c’ and angle ‘g’ is between sides ‘a’ and ‘b’. The unit cell lengths a, b, c, may be assigned along x, y and z-axis, respectively but angles a, b and g have to be decided accordingly. The choice of x, y, z may be along any of the three-axis. These six parameters of the unit cell are called unit cell dimensions or crystallographic elements.

 

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