Solid State (Crystallography)
Out of three state of matter, liquids and gases are fluids and posses fluidity on account of their molecular motion. On the other hand solid state has fixed positions of constituent ions (in ionic solids) or fixed positions of atoms (in molecular solids) and these atoms or ions can only oscillate about their position.
The chapter includes the studies on the geometry (the external form and internal structure) of crystals and crystalline substances, i.e., solids, their development, growth as well as related properties.
Solids are characterized by their high density, low compressibility, definite shape, considerable mechanical strength and rigidity. These properties are due to the existence of very strong inter molecular/interatomic/interionic forces of attraction amongst the molecules, atoms or ions of the solids. These strong forces do not allow motion and confer rigidity and long range orders in solids.
Classification of Solids
Solids are divided into two classes, namely crystalline and amorphous solids. A solid is said to be crystalline if the constituents atoms or ions are arranged in a very regular and long range order fashion in a three dimensional. On the other hand, in amorphous solids, the arrangement of building constituents is not regular (short range order).
Difference between Crystalline and Amorphous Solids:
|Crystalline solids||Amorphous solids|
|1. They have definite and regular geometry due to definite and orderly arrangement of atoms, ions or molecules in three dimensional space.||1. They do not have any pattern of arrangement of atoms, ions or molecules and, thus do not have any definite geometrical shape.|
|2. They have sharp melting points and change abruptly into liquids.||2. Amorphous solids do not have sharp melting points and do not change abruptly into liquids.|
|3. Crystalline solids are anisotropic. Some of their physical properties are different in different directions.||3. Amorphous solids are isotopic. Their physical properties are same in all direction.|
|4. These are considered as true solids.||4. These are considered pseudo solids or super cooled liquid.|
|5. Crystalline solids are rigid and their shape is not distorted by mild distorting forces.||5. Amorphous solids are not very rigid. These can be distorted by bending or compressing forces.|
|6. Crystals are bound by plane faces. The angle between any two faces is called interfacial angle. For a given crystalline solid, it is a definite angle and remains always constant no matter how the faces develop. When a crystalline solid is hammered, it breaks up into smaller crystals of the same geometrical shape.||6. Amorphous solids not have well defined planes. When an amorphous solids is broken, the surfaces of the broken pieces are generally not flat and intersect at random angles. (Amorphous solids do not have any symmetry)|
|7. Example: NaCl, KCl, Sugar, Quartz etc.||7. Example: Plastic, Glass, Rubber etc.|
Note: Solids are also classified as true solids and pseudo solids. A pseudo solid is more easily distorted by bending and compressing forces. It may even tend to flow slowly under its own mass and lose shape. The rigidity and shape of pseudo solids are only apparent. These do not melt sharply, and are gradually softened over a wide range of temperature and eventually lapse into a liquid state. Pitch and glass are referred to as pseudo solids. On account of above characteristics, the term pseudo solids have now been replaced b y super cooled liquids. Thus, pitch and glass are now called super cooled liquids.
Classification of Solid Crystals on the basis of nature of bonds
The attractive forces which hold together atoms or molecules of a substance in form of groups are called bonds. The forces that hold crystals particles together are generally of five types and thus crystals can be classified into five t ypes according to be types of bonds which hold the constituent particles of the crystals because of these forces.
(a) Ionic Crystals: Ionic solids are built from the mutual attractions of cations and anions. The lattices in ionic crystal consist of alternate positive and negative ions in equivalent amount arranged in a order so that the potential energy of the ions in the lattice is minimum. The forces which bind the constituent ions are electrostatic in nature and are strong forces. Such crystals are normally found in ionic compounds, e.g., NaCl, MgCl2, KCl, etc.
(b) Molecular Crystals: Molecular solids are assemblies of discrete molecules held in place by intermolecular forces. In molecular crystals, the repeating unit is chemically identifiable atoms or molecules which do not carry a net charge. Molecular solids are further divided into three categories.
(c) Covalent Crystals: Covalent solids or network solids consist of atoms covalently bonded to their neighbours throughout the extent of solid. In covalent crystals, atoms at their lattice point are held together by shared pairs of electrons between them. The covalent bonding extends throughout the crystals in spatial direction and has no small molecules in the conventional sense. The continuous, directional and rigid bonding in the lattice makes the whole crystal a single large molecule. Pure covalent solids are good insulators because all the valence electrons are tightly held in the covalent bonding. However, in presence of certain impurities, they become reasonably good conductors and are known as semiconductors.
Since covalent bonds are relatively weaker than ionic bonds and therefore covalent solids have relatively lower m.pt. However, covalent solids with giant molecular structure possess very high m.pt. e.g., carbon; the two crystalline polymorphous of carbon are diamond and graphite.
(d) Metallic Crystal: In metallic crystals, the lattice consists of assembly of positive ions immersed in a sea of mobile electrons. The binding force is due to:
(i) Attraction between positive ions or ion cores (consisting of nucleus and non-valence electrons) of the metal and electron cloud of valence electrons,
(ii) The mutual repulsion of free electrons,
(iii) The mutual repulsion of ion cores.
The electrons cloud of valence electrons can migrate throughout the crystal lattice and the gives rise to high electrical conductance, metallic lustre and thermal conductivity, etc.