The following are the characteristic properties of the solid state:
(i) They have definite mass, volume and shape.
(ii) Intermolecular distances are short.
(iii) Intermolecular forces are strong.
(iv) Their constituent particles (atoms, molecules or ions) have fixed positions and can only oscillate about their mean positions.
(v) They are incompressible and rigid.
Solids can be broken down into two major categories: crystalline solids and amorphous solids.
Amorphous solids, such as glass, are those solids that do not have any underlining order in their molecular structure. On the other hand, crystalline solids are those solids that have a well defined order in their structure.
Crystalline solids can be further broken down into four groups: ionic solids, molecular solids, network solids and metallic solids. Ionic solids are those solids in which the individual atoms form bonds as a result of positive and negative charges on the atoms.
Crystalline substances can be described by the types of particles in them and the types of chemical bonding that takes place between the particles.
There are four types of crystals:
(1) Ionic: An ionic crystal is a crystalline ionic compound. They are solids consisting of ions bound together by their electrostatic attraction into a regular lattice. Examples of such crystals are the alkali halides, including potassium fluoride, potassium chloride, potassium bromide, potassium iodide, sodium fluoride.
(2) Metallic: Metallic crystals consist of metal cations surrounded by a "sea" of mobile valence electrons. Covalent crystals are composed of atoms which are covalently bonded to one another. Molecular crystals are held together by weak intermolecular forces.
(3) Covalent or network: A network solid or covalent network solid (also called atomic crystalline solids) is a chemical compound (or element) in which the atoms are bonded by covalent bonds in a continuous network extending throughout the material.
(4) Molecular: Molecular crystals are substances that have relatively weak intermolecular binding, such as dry ice (solidified carbon dioxide), solid forms of the noble gases (e.g., argon, krypton, and xenon), and crystals of numerous organic compounds.
The crystal lattice is the symmetrical three-dimensional structural arrangements of atoms, ions or molecules (constituent particle) inside a crystalline solid as points. It can be defined as the geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space.
Primitive Unit Cell: When particles in unit cell are present only at the corners, it is called the primitive unit cell.
Centred Unit Cells: When particles are present at other positions in addition to those at corners in a unit cell, it is called a Centred Unit Cell. There are 3 types of Centred Unit Cells:
(1) The number of atoms contained in one simple cubic unit cell of monoatomic substance is 1. 8 atoms are present at 8 corners of a simple cubic unit cell. Each atom contributes one eigth to the unit cell. Total contribution =8×81=1
(2) The number of atoms contained in one face-centred cubic unit cell of monoatomic substance is 4. 8 atoms are present at 8 corners of a fcc unit cell. Each atom contributes one eigth to the unit cell. Total contribution =8×81=1. 6 atoms are present at 6 corners of a fcc unit cell. Each atom contributes one half to the unit cell. Total contribution =6×21=3. Total number of atoms in one fcc unit cell =1+3=4.
(3) The number of atoms contained in one body-centred cubic unit cell of monoatomic substance is 2. 8 atoms are present at 8 corners of a bcc unit cell. Each atom contributes one eigth to the unit cell. Total contribution =8×81=1. 1 atom is present at body centre of a bcc unit cell. The atom contributes fully to the unit cell. Total contribution =1×11=1. Total number of atoms in one fcc unit cell =1+1=2.
The term "closest packed structures" refers to the most tightly packed or space-efficient composition of crystal structures (lattices). To maximize the efficiency of packing and minimize the volume of unfilled space, the spheres must be arranged as close as possible to each other.
In one dimension close packing, spheres are arranged in a row such that adjacent atoms are in contact with each other. Coordination number is defined as the no. of the nearest neighbour particles. In case of one dimension close packing, coordination number is equal to two.
In two-dimensional close packing, a row of closed packed spheres are stacked to obtain a two-dimensional pattern. This stacking is done in two ways: Square close packing: The second row can be placed exactly below the first row in a close packing.
Formation of three-dimensional close packing can be done by placing the second square closed packing exactly above the first one. In this close packing, the spheres are aligned properly in horizontally and vertically. Similarly, by placing more layers one above the other, we can obtain a simple cubic lattice.
Number of octahedral voids = Number of close-packed particles
Number of tetrahedral voids = 2 × Number of close-packed particles
In ionic solids, the bigger ions (usually anions) form the close-packed structure and the smaller ions (usually cations) occupy the voids.
If the latter ion is small enough, then it occupies the tetrahedral void, and if bigger, then it occupies the octahedral void.
Not all the voids are occupied. Only a fraction of the octahedral or tetrahedral voids are occupied.
The fraction of the octahedral or tetrahedral voids that are occupied depends on the chemical formula of the compound.
Packing Efficiency
Packing efficiency is defined as the percentage of space occupied by constituent particles packed inside the lattice. It can be calculated with the help of geometry in three structures namely: HCP and CCP structures, Body-Centred Cubic Structures.
Packing efficiency can be written as below, Packing efficiency = Volume occupied by 6 spheres ×100 / Total volume of unit cells. Examples are Magnesium, Titanium, Beryllium etc. In body-centered cubic structures, the three atoms are arranged diagonally.
Both ccp and hcp are highly efficient lattice; in terms of packing. The packing efficiency of both types of close packed structure is 74%, i.e. 74% of the space in hcp and ccp is filled. The hcp and ccp structure are equally efficient; in terms of packing. The packing efficiency of simple cubic lattice is 52.4%.
In body-centred cubic unit cell (BCC) has atoms at each corner of the cube and an atom at the centre of the structure. The packing efficiency of the BCC lattice is 68%.
In a simple cubic lattice, the atoms are located only on the corners of the cube. Since a simple cubic unit cell contains only 1 atom. The packing efficiency of the simple cubic cell is 52.4 %. Thus 47.6 % volume is empty space (void space) i.e. almost half the space is empty.
Edge length of a unit cell of a cubic crystal = a,
Density of the solid substance = d
Molar mass = M
Volume of a unit cell = a3
Mass of the unit cell = number of atoms in unit cell × mass of each atom = number of atoms present in one unit cell (z) × mass of a single atom (m)
Any irregularity in the pattern of crystal arrangement in a solid lattice is called imperfection in solids. The occurrence of defects takes place when crystallization (the process of formation of crystals) occurs at a very fast or at an intermediate rate. This is because particles don’t get enough time to arrange themselves in a regular pattern. Basically, defects fall out in two forms:
Point Defect: Point Defect occurs when an atom is missing or is irregularly arranged in a crystal lattice. In other words, the deviations and irregularities observed around an atom or a point are known as a Point Defect. Point Defects are classified into two defects, namely:
Line Defect: Line defects, or dislocations, are lines along which whole rows of atoms in a solid are arranged anomalously. The resulting irregularity in spacing is most severe along a line called the line of dislocation. Line defects can weaken or strengthen solids.
Electrical properties are their ability to conduct electrical current. Various electrical properties are resistivity, Electrical conductivity, temperature coefficient of resistance, dielectric strength and thermoelectricity.
Electrical conductivity in metals is a result of the movement of electrically charged particles. It is these "free electrons" that allow metals to conduct an electric current. Because valence electrons are free to move, they can travel through the lattice that forms the physical structure of a metal.
Electrical conductivity of semiconductors increases with increase in temperature, since more electrons can jump to the conduction band due to small gap between the valence band and conduction band. Silicon and germanium exhibit this behavior and are called intrinsic semiconductors.
Anything that is magnetic, like a bar magnet or a loop of electric current, has a magnetic moment. A magnetic moment is a vector quantity, with a magnitude and a direction. An electron has an electron magnetic dipole moment, generated by the electron's intrinsic spin property, making it an electric charge in motion.
On the basis of their magnetic properties, substances can be classified into five categories: (i) paramagnetic (ii) diamagnetic (iii) ferromagnetic (iv) antiferromagnetic and (v) ferrimagnetic.
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