Molecular structure of a material
Matter is made of molecules and atoms. An atom consists of nucleus and electron. The nucleus contains protons and neutrons, collectivity called nucleons. Nuclear forces operating between different nucleons are responsible for the structure of the nucleus. Electromagnetic forces operate between a pair of electrons and between an electron and the nucleus. These forces between the molecules are responsible for the structure of the material.
Interatomic and intermolecular forces
The force between two atoms can be typically represented by the potential energy curve shown in the zero of potential energy is taken when the atoms are widely separated (r----->∞). The minimum potential energy indicates equilibrium position (r = r0). Note that inter atomic force is a result of attractive and repulsive forces. When r < ro repulsive force dominates. For r > r0 attractive force dominates. At r = r0 the attractive and repulsive forces are balanced.
Therefore at r = r0. du / dr = 0
Attractive forces ∝1 / r7 and repulsive forces are proportional to 1 / r13
That is, u = A / r12 – B / r6
Repulsive force contribution is, F = du / dr and attractive forces contribution is = + 12A / r13 – 6B / r7
Force between identical molecules is called cohesive force and force between dissimilar molecules is called adhesive force.
The range of molecular force is of the order of 10-10m. A sphere around a molecule having a radius equal to the range of molecular force is called sphere of molecular action of that molecule.
Bonds: - the electromagnetic interaction between the electrons and nuclei result in bonds. Logic or electrovalent bonding, covalent bonding and metallic bonding are some important examples.
Regarding separation between atoms in solids it is of the order of r0. in liquids atomic separation is slightly greater than r0. and in gases atomic separation >>r0.
In solids the intermolecular forces are so strong that the molecules or ions stay fixed at their equilibrium three dimensional arrangement, we call such solids as crystals. Te positions occupied by the molecules or the ions are called lattice points. Crystalline solids may be divided into four categories depending upon the nature to the bonding between the basic units.
Molecular solids: - molecules are formed due to covalent bonds between the atoms. The bonding between the molecules depends on whether the molecules are polar or no polar. If the centre of negative change coincides with the centre of positive charge. The molecule is called no polar. Molecules of H2, O2, and CI2and so on are no polar; otherwise the molecules are called polar. For example water molecule is polar. Bond between polar molecules is called dipole-dipole bond and the bond between no polar molecules is called a van deer wall’s bond. Molecular solids are usually soft and have low melting point. They are poor conductors of electricity.
Ionic solids: - in such a solid lattice points are occupied by positive and negative ions. Electrostatic force between the ions bonds the solid. Since these attractive forces are quite strong, these materials are usually hard and have high melting point, they are poor conductor of electricity. However, in aqueous solution form or in molten form they conduct electricity
Covalent solider: - atoms are arranged in the crystalline form the neighboring atoms are bound by share electron, large solid structures are possible, silicon, diamond and so on are examples. These are quite hard and poor conductor of electricity.
Metallic solids: - positive ions are situated at the lattice sites. The lions are formed by detaching one or more electrons form the constituent atoms. These electormsare highly mobile and move throughout the solid just like a gas. They are very goods conductors of electricity.
Amorphous or glassy solids: - many solids do not exhibit long range ordering. They only show local ordering four or five molecules are bonded together to form a structure. Such independent units are randomly arranged just like liquids. However, intermolecular forces in amorphous solids are much stronger than those in liquids. This presents the amorphous solid to flow like a liquid. These solids do not have a well defined melting point.
Homogeneous bodies having similar properties in all directions are called isotropic. Heterogeneous bodies having different properties in different directions are termed anisotropic.
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