Structure of Solid Materials | Engineering

Materials differ from one another because of the differences in their properties. Differences in properties of various materials are due to the differences in their structure. Different materials possess different structures. The structure of material exhibits its internal and surface details. These details can be examined and expressed in different orders of magnification varying from a few times to several thousands.

In order of decreasing magnification, the structure of a solid material can be expressed as follows:

i. Atomic Structure

ii. Crystal structure

iii. Micro Structure

iv. Macro Structure

All solid materials consist of a large number of particles called molecules which are bound together to form the bulk material. Each molecule is further composed of tiny particles called atoms. Individual properties of atoms and their arrangement in the molecule determine the properties of the material. Therefore, to understand the properties and structure of materials, it is necessary to start with the structure and characteristics of individual atoms.

1. Atomic Structure of Solid Materials:

An atom of any element consists of the nucleus and the electrons. The nucleus is a stationary mass, situated at the centre and carrying a net positive charge. It consists of heavy particles—protons and neutrons. Each of these particles is 1836 times heavier than an electron. Each proton carries a positive charge while the neutron carries no charge.

The electrons revolve around the nucleus in definite orbits. They are bound to the nucleus by different energy levels. An electron has a very small mass, and carries a net negative charge. In its normal state an atom carries equal number of electrons and protons, and is therefore electrically neutral. Electrons belonging to an atom may be classified as inner and outer electrons.

The outer electrons are those which are least firmly bound to the atomic nucleus and revolve in the outer orbitals. They are also called valence electrons. The inner ones are more firmly bound to the atomic nucleus and revolve in the inner orbits. They are also called the core electrons. Since the valence electrons are not bounded firmly by the nucleus they can be easily removed from the atom. When this happens, the atom becomes a positive ion.

Each electron performs two types of motion simultaneously. One motion is revolution around and nucleus in definite orbits. The other is spinning on its own axis. The electrons never fall into the nucleus by the electro-static attraction between the two differently charged particles, and are held in their own orbits. This is because the electrostatic force of attraction is balanced by the kinetic energy of rotation and the potential energy of the electron.

In an atom the number of electrons in a given orbit and the motion they execute are fixed. Pauli discovered that a single electronic orbit can contain no more than two electrons at the same time, and that these two electrons must spin in opposite directions. Thus in all elements, whose atomic numbers are greater than 2, there exists more than one orbit.

A moving particle has wave properties associated with it. Thus a moving electron is associated with a wavelength given by the de-Broglie equation-

λ = h/p = h/mv

where, λ = Wave length of the moving electron

p = Momentum of the electron

m = Mass of the electron

v = Velocity of the electron

h = Planck’s constant.

Later, Einstein found that a definite packet of energy, called one quantum of energy, is involved in a unit atomic process. The magnitude of a quantum of energy is given by-

E = hv

Where,

E = Magnitude of a quantum of energy

v = Frequency of radiation

The equation states that there is one to one relationship between the frequency of radiation and the magnitude of energy packets. The frequency of radiation is related to its wavelength by the following equation-

v = c/λ

where,

c = Velocity of light

λ = Wavelength of radiation.

Energy Levels and Quantum Numbers:

The electrons in an atom are arranged in different shells, known as K-shell, L-shell, M-shell, etc. Each shell includes a fixed number of orbits. These shells are further divided into subshells depending upon the total number of electrons in each shell. Each subshell (orbit) is at a definite distance and the nucleus exerts a definite force on the electrons in this subshell.

This force is known as the energy of the orbit. Each subshell which can be occupied by the electrons is called energy level in the atom. Each electron of the atom belongs to a particular orbit and hence occupies a definite energy level. The total energy of the atom is the sum of the energy levels occupied by different electrons.

The arrangement of electrons in different orbits is such that it produces a minimum total energy in the atom. This arrangement of electrons becomes the most stable state of the atom. If an electron occupies a higher energy orbit, it is called an excited state of the atom.

Various shells, the subshells and the number of electrons which can occupy a given subshell are shown in F 2.1.

Each electron in the atom occupies a particular shell and subshell. The exact position of the electron in the atom is specified by quantum numbers. There are four quantum numbers. In an atom, no two electrons can have all the four quantum numbers same at any time.

These four quantum numbers are as follows:

1. Principal quantum number ‘n’ which is a measure of the energy of the main shell. The electron shells are designated as K, L, M, N, O … corresponding to value of principal quantum number n = 1, 2, 3, 4, 5 … respectively. The number of electrons in any shell is given by 2n² e.g. the number of electrons in L-shell are 8 only.

2. Second quantum number, ‘l’ which is a measure of the angular momentum of the electron. The value of ‘l’ is useful to calculate the maximum number of electrons in a subshell which is equal to (2l + 1) e.g. the number of electrons in a sub-shell corresponding to l = 0 are 2. Moreover, the sub-shells in each main shell are designated by small alphabets s, p, d, f … corresponding to quantum number ‘l’ = 0, 1, 2, 3, … respectively.

3. Third quantum number ‘m₁‘, which is a measure of the component of the angular momentum in particular direction.

4. The fourth quantum number, ‘m_s‘ which determines the direction of spinning of the electron.

An electron having all the four different quantum numbers constitutes a quantum state. The various quantum states of the electrons in the first three shells of the atom are shown in the Table 2.2.

2. Crystal Structure of Solid Materials:

A crystal is a solid whose constituents are arranged in a systematic manner. The distinctive features of solids are their definite shape, their hardness and their fixed volume.

These characteristics can be explained on the basis of following facts:

(1) The constituents units (atoms, ions or molecules) of solids are held very close to each other so that the packing of the constituents is very efficient, consequently solids have high densities.

(2) Since the constituents of the solids are closely packed, it imparts rigidity and hardness to the solids.

(3) The constituents of solids are held together by strong forces of attraction. This results in their having definite shape and fixed volume.

Information regarding the above mentioned characteristics in solids can be obtained by the study of crystal structure, i.e., internal arrangement of atoms, ions or molecules in space. We may consider solids to exist in two different states, namely, amorphous and crystalline.

An amorphous solid is a substance whose constituents do not possess an orderly arrangement. The size of ordered region in these substances is limited to a few molecule distances. Amorphous solids are also called non-crystalline solids. A crystalline solid is a substance whose constituents possess an orderly arrangement in a definite geometric pattern.

A crystalline solid can be either a single crystal, where the solid consists of only one crystal or an aggregate of many small crystals or grains which are highly ordered crystalline regions of irregular size and orientation. The second type of crystalline materials is known as polycrystalline materials. Single crystals have long range order.  

3. Micro Structure of Solid Materials:

The appearance of the structure of a material under a microscope is called microstructure. Optical microscopes are used for magnifications upto 1000 times while electron microscopes can produce magnifications upto several thousand times. Microstructure of a material consists of phase structure and grain structure.

The phase structure is expressed in terms of various phases present, their relative amounts, distribution and alignment. Depending upon the number of phases present, microstructures are either single phase or multiphase structures. The grain structure of materials shows shape and size of the grains (crystals) which form the bulk material. It is characterised by grain boundaries, grain shape and size.

4. Macro Structure of Solid Materials:

The appearance of the structure of a material with the naked eye or by using a small hand magnifying lens is called macrostructure. Macro structural examination of a material is done to reveal the structural defects or in homogeneities of a large area. The method requires polishing and chemical etching of the surfaces to be examined.