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.