Structure of solids
Matter exists in nature in three states, namely solid, liquid and gas. The atoms, ions or molecules of matter are held together by the force of attraction. These attractive forces which hold constituent particles of substance together are known as binding forces or bonds. These bonds are quite weak in case of liquids and gases but quite strong in case of solids. If a solid is heated, it does not change its shape easily, which simply indicates that particles of solid are bound together by very strong binding forces.
The solids are characterized by a definite shape as well as a definite volume. Depending upon the nature and arrangement of atoms or molecules, solids are broadly classified into the following two categories:
1. Crystalline solids
2. Amorphous solids, non-crystalline or glassy.
Crystalline solids
Crystalline solids are those solids in which the atoms, molecules or ions, which constitute the solids, are arranged in a definite pattern in three dimensions throughout the solid. Rock salt, calcite, sugar, mica, quartz etc. are some examples of crystalline solids. Crystalline solids posses the following characteristics:
1. The atoms and molecules are arranged in a definite order, giving them a definite geometrical form.
2. They exhibit long range order.
3. They are bound by a flat surface.
4. They have homogeneous composition.
5. They have sharp melting point and boiling point.
6. They are anisotropic.
7. All the bonds in the crystalline solids are equally strong.
Amorphous solids
Amorphous solids are those solids in which the atoms are not arranged in regular geometrical pattern. Ordinary glass, sulfur, plastics etc. are examples of amorphous solids. Amorphous solids posses the following characteristics:
1. They do not possess characteristic geometrical shapes.
2. Atoms or molecules of amorphous solids are arranged in irregular ways.
3. They exhibit short range order.
4. Their composition may not be homogeneous.
5. They are isotropic, so they have same physical properties in all direction.
6. They do not have a sharp melting point.
7. Bonds between different atoms are not identical.
Energy bands in solids.
According to Bohr's theory, the electrons in an isolated atom have some well-defined energy states known as energy levels. An electron in an atom can be present in one of these permitted energy states only. However, when another atom is brought near the first, their outer electrons interact with one another and due to this interaction, permitted energy states of these outermost electrons are slightly modified. In other words, the permitted energy states of a single electron breaks up into the nearby permitted states. This interaction however leaves the inner shells of the atoms unaffected. Due to the interaction of atoms, the energy levels of the outermost shells are slightly modified.
For two atoms, one permitted energy state breaks up into two nearby permitted states. The number of these nearby permitted states is the same as the number of interacting atoms. In a solid, there are very large numbers of interacting atoms, about 10 atoms in one mole. Thus, each permitted energy levels in the outermost shell split up into 10 levels lying close together. Some of they are defined under the principle of physics. A well known definition is under the source value. They cannot be seen with our necked eyes. Love you paid for articles. Please increase the rate of our paid views.
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