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4 LIQUIDS AND SOLIDS eLearn.Punjab

4.7.4. Metallic Solids

In order to explain properties of

metallic solids various theories have been

proposed. A few of them are mentioned

here.

The first theory of metallic bonding is

called electron pool or electron gas theory.

This theory was proposed by Drude and

extended by Loren (1923). According to

this theory, each atom in a metal crystal

loses all of its valence electrons. These

valence electrons form a pool or a gas.

The positively charged metal ions are

believed to be held together by electron

pool or gas. These positively charged ions

occupy definite positions at measurable Fig (4.20) Positive ions surrounded by mobile electrons
distances from each other in the crystal

lattice. Valence elect rons are not attached to any individual ion or a pair of ions rather belong to

t he crystal as a whole. These electrons are free to move about from one part of the crystal to the

other. The force, which binds a metal cation to a number of electrons within its sphere of- influence,

is known as metallic bond. The following Fig. (4.20) gives an idea of electron gas model.

L. Pauling has tried to explain the metallic bond according to valence bond theory. According to

this theory, the metallic bond is treated essentially as covalent in character. However, it is assumed

that the covalent bonds are not localized but are highly delocalized in metal structure.

Recently, molecular orbital theory was applied to explain the characteristics of metallic

solids. According to this theory, it is assumed that the electrons in the completely filled orbitals

are essentially localized, while atomic orbitals containing the valence electrons interact or overlap

to form a set of delocalized orbitals. These delocalized orbitals are the molecular orbitals which

extend over the entire crystal lattice. Such a combination of atomic orbitals produce as a large

number of closely spaced states. These states of energy are also known as bands of energy. That is

why it is also called a band theory. The energy gap between two bands determines the properties

of the metallic solids.

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