Properties of Solids

A solid is a state of matter that has a definite volume and a definite shape, consisting of particles that are closely packed and move only by vibrating. Molecules in a solid do not move past one another. Solids are divided into two types: amorphous and crystalline. An amorphous solid is composed of particles that are not organized into a crystalline pattern. Rubber, glass, and plastic are examples of amorphous solids. A crystalline solid is a solid in which the particles repeat at regular intervals in an ordered crystalline lattice. Crystalline solids are further classified into four categories: metallic, ionic, covalent, and molecular. A metallic solid is composed of atoms held together by metallic bonds. Examples of metallic solids are pure metals, such as aluminum and copper, and alloys, such as steel and brass. Metallic solids are highly conductive to thermal energy and electrical energy, and they tend to be malleable (bendable without breaking), ductile (able to be drawn into a wire), and strong.

An ionic solid is a solid held together by electrostatic forces between positively and negatively charged ions. Sodium chloride (NaCl), known as table salt, is an example of an ionic solid. Ionic solids tend to have high melting points. They are also quite brittle, meaning they shatter easily. In solid form, ionic solids are poor conductors of electrical energy.

A covalent solid is a crystalline solid formed from a network of atoms joined by covalent bonds. Examples of covalent solids include diamonds and sugar. The arrangement of atoms in the network can vary widely depending on the molecules, resulting in a large variety of structures and properties. Carbon forms many different types of covalent solids (allotropes), including graphite and carbon nanotubes, which have important industrial applications.

A molecular solid is a crystalline solid held together by van der Waals intermolecular forces. For example, ice is a molecular solid. The attraction between the individual water molecules enables water to maintain its physical state. Molecular solids tend to have low melting points because of the ease of breaking the bonds between the molecules. They also tend to be poor conductors of electricity. Because crystalline solids are defined by repeating patterns of atoms, they are also subject to defects. Crystal defects are of three varieties: point defects, line defects, and planar defects.

A point defect occurs when a single atom of a crystal is incorrect. One type of point defect is a vacancy, a defect in which an atom is missing from a crystal. Another type of point defect happens when an ion shifts from its original point to an interstitial space, for example, a space between atoms. Impurities, or foreign material, can also constitute point defects.

Line defects occur when an entire line of atoms in a crystal is arranged irregularly. Line defects can weaken or strengthen a crystal, depending on the molecular arrangement of both the crystal and the defect. In metals, line defects weaken the strength and elasticity of the material and are responsible for mechanical failures.

Planar defects exist along the boundaries of surfaces within a crystal. They can arise when the orientation of atoms in one plane runs in a different direction to atoms in another plane. These areas of defects are on the boundary between the mismatched planes and have a high mobility, diffusivity, and chemical reactivity because they are less dense. This is particular problem when welding metals. Planar defects can cause cracks and shape imperfections in metals.