Types and Properties of Solids
Types of Solids
There are four types of crystalline solids depending upon the type of bond present in them.
(i) Ionic Solids
(ii) Covalent Solids
(iii) Molecular Solids
(iv) Metallic Solids
Properties of Ionic Solids
(i) Definition: The crystalline solids in which the particles forming the crystal are positively and negatively charged ions which are held together by strong electrostatic forces of attraction (ionic bond) are called ionic solids. The crystals of NaCl, KBr etc. are ionic solids.
(ii) Physical State: The cations and anions are arranged in a well defined pattern, so they are crystalline solids at room temperature. Under ordinary conditions of temperature and pressure they never exist in the form of liquids or gases.
(iii) Hardness, Volatility, Melting and Boiling Points: Ionic crystals are very stable compounds. Very high energy is required to separate the cations and anions from each other against the forces of attraction. That's why ionic solids are very hard, have low volatility and high melting and boiling points.
(iv) Nature of Ionic Solids: Ionic solids do not exist as individual neutral independent molecules. The cations and anions attract each other and these forces are non-directional. The close packing of the ions enables them to occupy minimum space. A crystal lattice is developed when the ions arrange themselves systematically in an alternate manner.
(v) Radius Ratio: The structure of ionic crystals depends upon the radius ratio of cations and anions.
Radius ratio = r+/r-
In NaCl; Na⁺ = 95 pm, Cl⁻ = 181 pm
Radius ratio = 95/181 = 0.525
NaCl and CsF have the same geometry because the radius ratio in both the cases is the same.
Properties of Molecular Solids
(i) Definition: Those solid substances in which the particles forming the crystals are polar or non-polar molecules or atoms of a substance are called molecular solids. Ice, sugar, Iodine (I2) and phosphorus (P4) are examples of molecular solids.
(ii) Regular Arrangement: X-rays analysis has shown the regular arrangement of atoms in the constituent molecules of these solids and we get the exact positions of all the atoms.
(iii) Softness: The forces, which hold the molecules together in the molecular crystals, are very weak. So they are soft and easily compressible.
(iv) Volatility, Melting and Boiling Points: They are mostly volatile and have low melting and boiling points.
(v) Conduction, Solubility and Density: They are bad conductors of electricity, have low densities and sometimes transparent to light. Polar molecular crystals are mostly soluble in polar solvents, while non-polar molecular crystals are usually soluble in non-polar solvents.
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