Ch 11The Solid State — Class 12 Chemistry, concept-first.

In Class XI you learnt that matter exists in three physical states — solid, liquid and gas. Which of these is the most stable for a given substance, at a particular temperature and pressure, is decide…

Solids are classified as crystalline or amorphous depending on the nature of the order in which their constituent particles are arranged.

Most solid substances are crystalline rather than amorphous. Metals such as iron, copper and silver; non-metals such as sulphur, phosphorus and iodine; and compounds such as sodium chloride, zinc sulp…

In molecular solids the constituent particles are molecules. They are sub-divided into three types according to the kind of attraction between the molecules.

In ionic solids the constituent particles are ions. The solid is a three-dimensional arrangement of cations and anions held together by strong coulombic (electrostatic) forces.

A metallic solid can be pictured as an orderly collection of positive metal ions immersed in a "sea" of free electrons.

When covalent bonds form continuously between adjacent atoms throughout a crystal, the whole crystal becomes effectively one enormous molecule.

Think of tiles laid to cover a floor: they generate a repeating pattern. If you mark the same point on every tile (say its centre) and look only at the marked points, you get a regular set of points —…

Unit cells fall into two broad categories — primitive and centred.

A crystal lattice is built from an enormous number of unit cells, and every lattice point holds one constituent particle.

A primitive (simple) cubic unit cell has atoms only at its eight corners. As Fig. 1.12 shows, each corner atom is shared between eight adjacent unit cells — four in the same layer and four in the laye…

A body-centred cubic (bcc) unit cell has an atom at each of the eight corners and, in addition, one atom right at the body centre. Fig.

A face-centred cubic (fcc) unit cell has atoms at all eight corners and also at the centre of each of the six faces. As Fig.

In solids the particles pack together as tightly as possible, leaving the least empty space. To understand how, we treat the particles as identical hard spheres and build the structure up in stages.

When particles close-pack into a ccp or hcp structure, two kinds of voids are created. The number of octahedral voids equals the number of close-packed particles (), while the number of tetrahedral vo…

However the constituent particles are arranged, some empty space always remains in the form of voids. The packing efficiency is the percentage of the total space that is actually occupied by the parti…

Both close-packed arrangements — hcp and ccp — are equally efficient, so it is enough to work out the efficiency for the ccp (fcc) structure.

In a body-centred cubic (bcc) structure the central atom touches the two corner atoms lying along the body diagonal (Fig. 1.25). Let the edge length be .

In a simple cubic lattice the atoms sit only at the corners of the cube and touch each other along the edge (Fig. 1.26). So the edge length and the radius are related by:

From the dimensions of a unit cell we can calculate its volume, and if the density of the substance is known we can find the mass of the atoms in the cell.

Even though crystalline solids possess long range order, real crystals are not perfect. A solid is usually an aggregate of many small crystals, and these carry defects — especially when crystallisatio…

Point defects are of three types: (i) stoichiometric, (ii) impurity and (iii) non-stoichiometric defects.

Solids show an astonishing spread of electrical conductivities — covering about 27 orders of magnitude, from roughly to . On this basis solids are grouped into three types:

A conductor may carry current either by the movement of electrons or by the movement of ions. Metallic conductors belong to the first category and electrolytes to the second.

In a semiconductor the gap between the valence band and the conduction band is small (Fig. 1.33c), so a few electrons can jump into the conduction band and give some conductivity.

Every substance has some magnetic character, and its origin lies in the electrons. Each electron behaves like a tiny magnet, with a magnetic moment arising from two motions (Fig. 1.35):