Question

Which type (or types) of crystalline solid is characterized by each of the following? (a) High mobility of electrons throughout the solid; (b) softness, relatively low melting point; (c) high melting point and poor electrical conductivity; (d) network of covalent bonds.

Short answer

(a) Metallic solids; (b) Molecular solids; (c) Ionic solids; (d) Covalent (network) solids.

Step-by-step solution

(a) High mobility of electrons throughout the solid

High mobility of electrons throughout the solid is characteristic of metallic solids. In metallic solids, the electrons are delocalized and form a "sea of electrons" which facilitate electrical conductivity and high thermal conductivity.

(b) Softness, relatively low melting point

Softness and relatively low melting point are characteristic of molecular solids. Molecular solids are held together by weak intermolecular forces, such as van der Waals forces, hydrogen bonding, or dipole-dipole interactions. Due to these weak forces, molecular solids are generally soft and have low melting points.

(c) High melting point and poor electrical conductivity

High melting point and poor electrical conductivity are characteristic of ionic solids. Ionic solids are made up of positively and negatively charged ions, held together by electrostatic (ionic) bonds. Ionic compounds have a crystalline lattice structure and high melting points due to the strong ionic bonds. However, since the ions are not free to move, ionic solids have poor electrical conductivity.

(d) Network of covalent bonds

A network of covalent bonds is characteristic of covalent (network) solids. Covalent solids consist of atoms bonded together by strong covalent bonds arranged in a network pattern. Examples of covalent solids include diamond and silicon dioxide (SiO2). These solids are generally very hard and have high melting and boiling points because of the strength of the covalent bonds.

Key concepts

Metallic Solids

When we talk about metallic solids, we are referring to materials that are typified by a unique electron configuration known as the 'metallic bond'. In these solids, the atoms relinquish their outer electrons, forming a sea of delocalized electrons that flow freely around the positively charged ionic cores.

This sea of electrons is the reason why metallic solids are good conductors of electricity and heat. The free electrons can move quickly in response to electric fields or thermal gradients, carrying charge and heat with them.

Furthermore, the nondirectional nature of metallic bonding contributes to the malleability and ductility of metals. This means that metals can be hammered into thin sheets (malleability) or drawn into wires (ductility) without breaking, which is ideal for various industrial applications.

Examples of metallic solids include copper, aluminum, and steel, each with their distinctive combination of strength, conductivity, and other metallurgical properties.

Molecular Solids

Molecular solids consist of atoms or molecules held together by intermolecular forces such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds. These forces are significantly weaker than the ionic or covalent bonds that hold together other types of solids.

Since the bonds are weaker, molecular solids tend to be softer and have lower melting and boiling points. This characteristic explains why many molecular solids, such as ice or dry ice, are easily changed from solid to liquid or gas at relatively low temperatures.

Besides softness, the weak forces in molecular solids mean they are typically poor electrical conductors. This is because they do not have free electrons or ions that can move throughout the structure. Sugar, solid carbon dioxide (dry ice), and ice are common examples of molecular solids.

Ionic Solids

Ionic solids are formed by the electrostatic attraction between positively and negatively charged ions. The high melting point of ionic solids is attributed to the strong coulombic (ionic) forces between the oppositely charged ions in their lattice.

Common table salt, sodium chloride (NaCl), is a classic example of an ionic solid. In sodium chloride, the sodium (Na+) and chloride (Cl-) ions are arranged in a repeated three-dimensional pattern, creating a crystalline structure that is stable and has a high melting point.

Despite their solid structure, ionic solids are poor conductors of electricity because the ions are locked in place and cannot flow freely. However, when melted into a liquid or dissolved in water, the ions are liberated and can conduct electricity, which is why saltwater is a conductor.

Covalent Network Solids

Covalent network solids are composed of a continuous network of covalent bonds joining atoms together throughout the material. Each atom is bonded to its neighbors in a way that forms a three-dimensional network, leading to substances that are extremely hard and have very high melting points.

One of the most well-known covalent network solids is diamond, where each carbon atom is tetrahedrally bonded to four other carbon atoms, creating a very strong and rigid structure. Silicon carbide (SiC) and quartz (SiO2) are two other examples, both prized for their durability and high melting points.

These solids do not conduct electricity well because all the valence electrons are tied up in the covalent bonds; there are usually no free or delocalized electrons available. Covalent network solids are often used in materials that require high heat resistance or durability.