The Nature of Solids

A Model for Solids The particles in solids are not free to move about. They tend to vibrate about fixed points. In most solids, the particles are packed against one another in a highly organized pattern. Solids tend to be dense and incompressible. Because of the fixed positions of their particles, solids do not flow.

A Model for Solids When you heat a solid, its particles vibrate more rapidly as their kinetic energy increases. The melting point (mp) is the temperature at which a solid turns into a liquid.

A Model for Solids At this temperature, the disruptive vibrations of the particles are strong enough to overcome the interactions that hold them in fixed positions. The melting and freezing points of a substance are at the same temperature.

Crystal Structure and Unit Cells Most solid substances are crystalline. In a crystal, the atoms, ions, or molecules that make up the solid substance are arranged in an orderly, repeating, 3-D pattern called the crystal lattice.

Crystal Structure and Unit Cells All crystals have a regular shape. The shape of a crystal reflects the arrangement of the particles within the solid. The type of bonding that exists between the atoms determines the melting points of crystals.

Crystal Structure and Unit Cells Ionic solids have high melting points. Molecular solids (covalently bonded) have low melting points. Not all solids melt. Wood and cane sugar decompose when heated.

Crystal Systems A crystal has sides, or faces. Crystals are classified into seven groups, or crystal systems that have the characteristic shapes. The seven crystal systems differ in terms of the angles between the faces and the number of edges of equal length on each face.

Crystal Systems The smallest group of particles within a crystal that retains the geometric shape of the crystal is known as a unit cell. The shape of a crystal depends on the arrangement of the particles within it. A crystal lattice is a repeating array of any one of fourteen kinds of unit cells.

Allotropes Some solid substances can exist in more than one form. Allotropes are two or more different molecular forms of the same element in the same physical state.

Allotropes Although allotropes are composed of atoms of the same element, they have different properties because their structures are different. – EX: Carbon – extended 3D carbon forms diamonds. Carbon stacked in sheets make graphite. Carbon atoms that form a hollow sphere form buckyballs.

Non-Crystalline Solids Not all solids are crystalline in form; some solids are amorphous. Amorphous solids lack an ordered internal structure because their atoms are randomly arranged. – EX: rubber, plastic, asphalt

Non-Crystalline Solids Other examples of amorphous solids are glasses – transparent fusion products of inorganic substances that have cooled to a rigid state without crystallizing.