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SOLIDS

CRYSTALLINE SOLIDS

AMORPHOUS SOLIDS

UNIT CELL and the geometrical pattern of points on which the unit cells are arranged is called a CRYSTAL LATTICE

Crystalline solids are ANISOTROPIC in nature, that is, some of their physical properties like electrical resistance or refractive index show different values when measured along different directions in the same crystals.

COVALENT SOLIDS

In molecular solids, the molecules are held together by dispersion forces, dipole-dipole forces, or hydrogen bonds. Even water, which can form strong hydrogen bonds, is a liquid at room temperature. With larger molecules, many weak attractions can combine to hold the molecules together.

IONIC SOLIDS

Remember that each ion in an ionic solid is surrounded by ions of opposite charge The type of ions and the ratio of ions deter-mine the structure of the lattice and the shape of the crystal. Ionic crystals are strong, but brittle. Repulsions between ions of like charge cause the crystal to shatter.

ATOMIC SOLIDS

It is the most diverse group; they will be divided in three subgroups.

Metallic solids

They consist of positive metal ions surrounded by a sea of mobile electrons. The strength of the metallic bonds between cations and electrons varies among metals and accounts for their wide range of physical properties. The mobile electrons make metals malleable and ductile. When force is applied to a metal, the electrons shift and thereby keep the metal ions bonded in their new positions.

Mobile electrons make metals, good conductors of heat and electricity. ELECTRON-SEA-MODEL, in which the valence electrons are delocalized and are visualized as being free to move throughout the metal. Metals are not covalently bonded, but the attractions between atoms are too strong to be Van der Waals forces. In metals, valence electrons are delocalized throughout the solid.

They typically retain a sea of mobile electrons and so remain conducting.

INTERSTITIAL ALLOYS: form between atoms of different radius.

SUBSITUTIONAL ALLOY: form between atoms of comparable radius, where one atom substitutes for the other in the lattice.

GROUP 18 SOLIDS

This group is known as noble gasses, but once cooled below their respective melting points, they form an ordered solid. Some examples include the use of solid argon to study highly reactive molecules, and solid neon to allow reaction and formation of xenon hydrides.

AMORPHOUS SOLIDS

An amorphous solid is one in which the particles are not arranged in a regular, repeating pattern.

It does not contain crystals.

They melt gradually so they do not have a specific melting point.

Amorphous solids are ISOTROPIC in nature, that is, some of their physical properties like electrical resistance or refractive index show the same values when measured along different directions in the same crystals.

Amorphous solids do not have a long-range order of arrangement of their constituent particles.

Amorphous solids do not have a sharp melting point but melts over a range of temperatures. Amorphous solid does not possess the characteristic heat of fusion.

Amorphous solids, when heated and then cooled slowly by annealing, becomes crystalline at some temperature.