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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Third Row Elements Page 2

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4.

Nature of Bonding in Crystal Lattice

 

Solids exist in two forms, crystalline and amorphous. In the amorphous form, the solid has definite shape and volume but the arrangement of atoms, molecules or ions is not regular. In the crystalline form, the solid has a definite shape and volume and also a regular arrangement of atoms, molecules or ions in them, thus they have a geometrical shape.

  • Crystal Lattice
    The orderly, geometrical, three-dimensional pattern of atoms, ions or molecules present in a crystal is called lattice.
  • Space Lattice
    The regular, three-dimensional pattern of atoms, molecules or ions present in a crystal represented as points in space is called space lattice.
  • Unit Cell
    The simplest three-dimensional recurring unit present in a crystal structure is called a unit cell
    Crystals are classified into four types depending upon the nature of units present in them.
    Metallic solids-units occupying the lattice points are cations surrounded by a pool of electrons.
    Network solids (Covalent solids) - Units occupying the lattice points are atoms.
    Molecular solids - Units occupying the lattice points are molecules.
    Ionic solids - Units occupying the lattice points are positively and negatively charge ions.

 

 

5.

Metallic Solids

 

  • Metallic solids have their characteristic metallic properties due to the presence of metallic bond
    Metallic bond may be defined as the force of attraction that binds metal cations to mobile electrons within its sphere of influence in a metal.
    Among the third row elements, Na, Mg and Al are metallic solids.

         Crystal Structure of Sodium
Sodium crystal is a body-centered cubic crystal. Each sodium ion is surrounded by eight equidistant sodium ions arranged at the corners of the cube which is a unit of the crystal. The coordination number of sodium crystal is eight.
Body-centered cubic structure is less compactly packed, atoms occupy 68% of the total space and as sodium has only one valence electron per atom, the metallic bond is a weak bond. Thus, sodium is a soft metal.

         Crystal of Magnesium
A magnesium crystal is a hexagonal closely packed crystal with each magnesium ion surrounded by twelve equidistant magnesium ions. The co-ordination number of magnesium crystal is twelve.
Atoms occupy 74% of the total space.
Magnesium has two valence electrons per atom, hence the metallic bond is stronger than sodium. Thus, it is harder than sodium.

         Crystal of Aluminium
An aluminium crystal is face-centred cubic crystal with each aluminium ion surrounded by twelve equidistant aluminium ions. The co-ordination number of aluminium crystal is twelve.
The open space present in the crystal is 26%.
Aluminium has three valence electrons per atom. Hence, the metallic bond is stronger than sodium and magnesium. Thus, aluminium metal is harder than sodium and magnesium metals.

 

 

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