Introduction:

A substance can exist as a solid, a liquid, or a gas. The state of a substance depends on the space between its particles and on the way in which the particles move. The particles in all matter are always in motion.

1) A solid is a substance which consists of a fixed volume and a fixed shape. In a solid, the particles are close altogether and might form a regular pattern. Particles in a solid are locked in place, however they vibrate. As each and every particle is attached to some others, individual particles can't move from one location to the other, and the solid is rigid.

2) A liquid consists of a fixed volume however doesn't encompass a fixed shape. Liquids take on the shape of the container they are in. The particles in a liquid are attracted to one other and are close altogether. Though, particles in a liquid are not fixed in place. They move freely adequate to collide and move past one other.

3) A gas consists of no fixed shape or volume. A gas can take on both the shape and volume of the container. Gas particles are not close to one other and can move simply in any direction. They often collide. There is much more space among gas particles than there is between particles in a liquid or a solid. The space between gas particles can raise or decrease with changes in pressure and temperature.

Classification of matter on the basis of composition:

On the basis of chemical composition matter can be categorized as:

1) Heterogeneous matter

2) Homogeneous matter

Heterogeneous matter:

A substance is said to be heterogeneous if it shows various properties at its different position. Different kinds of heterogeneous matter are as:

a) Suspension b) Colloid c) Heteromixture  

- Suspension: This is a heterogeneous mixture in which the solutes particles don't dissolve however remain suspended all through the bulk of the medium. These particles can be seen via the naked eye. The solute particles settle down whenever the suspension is left undisturbed. These can be separated from the mixture through the procedure of filtration.  

Example: dirt particles in water and soar milk.

- Colloid: Colloids are heterogeneous mixture of two components having the size of the particle of 1 nm to 100 nm (or 10 A^o to 1000 A^o). These particles of colloid are uniformly spread all through the solution. Due to comparatively smaller size of the particles as compared to the mixtures, it appears to be homogeneous; however in reality colloids are heterogeneous. These are possessing two phases: Dispersed phase and dispersion medium.

- Heteromixture: This is obtained via mixing of two or more substance in any ratio. These possess the mixed properties of the combined substance. These can be separated through the physical method.

Example: A mixture of sand and common salt.

Homogeneous matter:

A substance is a homogeneous matter if the smallest portion of it shows the similar physical and chemical properties. Example: Air, solution of sugar by water, an intimate mixture of two or more than two metals (that is, alloys).

Homogeneous matter can be categorized into two types: Homogeneous mixtures and pure substances.

- Homogeneous mixture: It is a mixture of two components which appears in a single phase is termed as homogeneous mixtures. These are known as solutions. The homogeneous mixture of solute and solvent are termed as solutions. These solutions are as well termed as true solutions or crystalloids.             

Example: Homogeneous mixture of water and sugar gives true solutions.

- Pure substance: These are made up of only one kind of particles like atoms or molecules. Further these are categorized as elements and compounds.

Element: Simple forms of matter that can't be decomposed to further simple substances are known as the elements. 118 elements are discovered till nowadays, out of which 92 are naturally occurring elements and remaining are artificially made elements. Example: Hydrogen, mercury, iron, gold and so on. Elements are further categorized as metals, non-metals and metalloids.

Compounds: The substances are made up of two or more than two elements combined by one other in a definite ratio via mass are known as compounds. These can be decomposed to its elements via chemical or electro-chemical reactions.

Classification of solids:

Most of the people who have lived in the world long adequate to read this have already made a rough way of classifying solids on the basis of macroscopic properties they can simply observe; everyone recognizes that a piece of metal is basically dissimilar from a rock or a chunk of wood.

Unluckily, the ingenuity of nature is far too versatile to fit to any simple system of categorizing solids, particularly those composed of more than a single chemical substance.

Classification according to bond type:

The most generally employed classification is based on the types of forces that join the molecular units of a solid altogether. We can generally differentiate four main categories on the basis of properties like general appearance, hardness and melting point.

Classification by type of molecular unit:

Solids, similar to the other states of matter, can be categorized according to whether their basic molecular units are atoms, electrically-neutral molecules, or ions. However solids have an additional property that gases and liquids don't: an enduring structural arrangement of their molecular units. Over-simplifying just a bit, we can sketch a rough categorization of solids according to the given scheme:

Crystalline solids:

In a solid comprised of similar molecular units, the most favored (that is, lowest potential energy) locations take place at regular intervals in space. If each of such locations is in reality occupied, the solid is acknowledged as a perfect crystal.

What really states a crystalline solid is that its structure is comprised of repeating unit cells each having a small number of molecular units bearing a fixed geometric relation to one other. The resultant long-range order states a (3-D) three-dimensional geometric framework termed as a lattice.

Fig: Crystal lattice

Geometric theory illustrates that only fourteen (14) various kinds of lattices are possible in (3-D) three dimensions, and that merely six different unit cell arrangements can produce these lattices. The regularity of external faces of crystals, which however correspond to lattice planes, reflects the long-range order inherent in the underlying structure.

Excellence is no more achievable in a crystal than in anything else; real crystals have defects of different types, like lattice positions which are either vacant or occupied via impurities, or through abrupt dislocations or displacements of the lattice structure.

Most of the pure substances, comprising the metallic elements, form the crystalline solids. However there are some significant exemptions.

Metallic solids:

In metals, the valence electrons are free to roam all through the solid, rather than being localized on one atom and shared by a neighboring one. The valence electrons act very much similar to a mobile fluid in which the fixed lattice of atoms is immersed. This gives the final in electron sharing, and makes a very strong binding effect in solids comprised of elements which encompass the requisite number of electrons in their valence shells. The characteristic physical properties of metals such as their capability to bend and deform devoid of breaking, their high electrical and thermal conductivities and their metallic sheen are all due to the fluid-like behavior of the valence electrons.

Molecular solids:

Remember that a molecule is stated as a discrete aggregate of atoms bound altogether adequately tightly (that is, via directed covalent forces) to allow it to retain its individuality whenever the substance is dissolved, melted and vaporized.

The two terms above in the preceding sentence are significant; covalent bonding means that the forces acting between atoms in the molecule are much stronger than those acting between the molecules and the directional property of covalent bonding presents on each and every molecule a distinguishing shape that influences a number of its properties. Most of the compounds of carbon and thus, most chemical substances, fall to this class.

Most of the simpler compounds as well form molecules: H₂O, NH₃, CO₂, and PCl₅ are well-known instances. A few of the elements, like H₂, O₂, O₃, P₄ and S₈ as well take place as discrete molecules. Solids and liquids which are comprised of molecules are held altogether via van der Waals forces, and most of their properties replicate this weak binding. Therefore molecular solids tend to be soft or deformable, encompass low melting points, and are often adequately volatile to evaporate (or sublime) directly to the gas phase; the latter property frequently provides these solids a distinctive odor.

Symmetry and Crystal System:

Crystal might be differentiated from one another by their external morphology that usually appears to encompass some regularity of arrangement. It is soon recognized that the regularity of external arrangement means a regularity of internal arrangement. This regularity is at times covered via the presence of some over-grown or destroyed faces. Firmly speaking, categorization of crystals is made up on the basis of their feature internal structure that possesses some definite symmetry. The symmetry of solid body might be explained in terms of the various elements of symmetry it possesses. Assume that we are looking at a cube and if for a moment we take our sight away from this cube and in the meantime someone rotates the cube via 90° around an imaginary axis passing via the centre of the top and bottom faces and if we look at the cube again, we will find out no change. This second orientation of crystal is closely identical to the first orientation. In a similar manner, the cube can be rotated two times more to get same orientations. Such a crystal that consists of more than one orientation in space and which are identical from other is stated to have symmetry. The imaginary axis regarding which this rotation operation has been executed is a symmetry element, termed as axis of rotation. In this case the crystal has been rotated via 90° and therefore it possesses a 360/90 = 4-fold axis of rotation. As a cube comprises of six faces, there will be three 4-fold axis of rotation. (Figure shown below)

Likewise, a cube might be brought into coincidence by itself by rotation via one third of a revolution (120°) about a body diagonal. Such an axis of symmetry is termed as 360/120 = 3-fold axis of symmetry. As there are four body diagonals in a cube, it consists of four 3-fold axis of rotation (figure illustrated below). The figure illustrates one of the six axes of 2-fold symmetry, emerging from the opposite edges of a cube. The one centre of symmetry of a cube is symbolized simply at the mass centre (that is, middle point) of a cube.

If we consider a hexagon, we notice that it can be brought into coincidence by itself by rotation via 60°. Therefore, a hexagonal crystal possesses 360/60 = 6-fold axis of rotation.

Fig: Symmetry and Crystal System

Seven Crystal Systems:

As solid materials are of numerous different shapes, it might appear at first sight that might be an infinite number of interfacial combinations. However this is not true. A careful assessment of some thousand crystals of different substances reveals that there are merely seven possible crystal symmetries represented by solids. Different solids that show the similar symmetry elements are all categorized as belonging to the similar system. This can be observing that each and every crystal system is determined on the presence of some rotation axes.

Fig: Seven Crystal Systems

Bravais Lattices:

However for simplicity we have so far selected to explain merely a two dimensional space lattice, the extension of these concepts to (3-D) three dimensions applies uniformly well. If the seven crystal systems illustrated, are symbolized by their primitive unit cells, then we shall encompass seven possible lattice types. However at times, the smallest primitive unit cell doesn't show the full symmetry of the lattice. In such a case, the unit cell having high symmetry is selected. For instance, we might take the face centered cubic structure (FCC) of silver.

In the face-centered representation, there are four silver atoms related with this unit cell. (Eight corner atoms = 8 x 1/8 = 1 atom; Six face atoms = 6 x 1/2 = 3 atoms; Net Total = 4 atoms.)

We might now select a primitive unit cell as illustrated by heavy lines. It includes only one silver atom per unit cell. However the representation consists of the smallest number of atoms per unit cell, however them it doesn't display the complete symmetry of the structure. The face-structure symbolizes cubic symmetry, while, the primitive cell unit cell symbolizes a rhombohedral. The unit cell for these reasons is selected as the face-centered cubic.

It was illustrated by A. Bravais in the year 1848 that all the possible three dimensional space lattice are of fourteen different types. These fourteen lattice types (as well known as Bravais lattices) are derived from seven crystal systems. The unit cells for such fourteen Bravais lattices are illustrated in the figure below.

Fig: Bravais Lattices

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