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 are characterized by high mobility of electrons throughout the solid, resulting in good electrical and thermal conductivity. (b) Molecular solids are characterized by softness and relatively low melting points due to weak intermolecular forces between molecules. (c) Ionic solids have high melting points and poor electrical conductivity, as they are made of positive and negative ions held together by strong electrostatic forces. (d) Covalent (or network) solids have a network of covalent bonds, making them very hard, with high melting points and poor electrical conductivity. Examples include diamond and quartz.

Step-by-step solution

(a) High mobility of electrons throughout the solid

: This property is characteristic of metallic solids. Metallic solids have a "sea of electrons" that are delocalized and can move throughout the entire solid. This high mobility of electrons gives them good electrical and thermal conductivity.

(b) Softness, relatively low melting point

: Softness and low melting points are properties of molecular solids. Molecular solids are formed by weak intermolecular forces (such as Van der Waals forces, hydrogen bonding, and dipole-dipole interactions) between molecules, which can be easily broken, resulting in the solid being soft and having a relatively low melting point.

(c) High melting point and poor electrical conductivity

: These properties are associated with ionic solids. Ionic solids are made of positive and negative ions held together by strong electrostatic forces (ionic bonds). Due to the strong ionic bonds, ionic solids have high melting points. However, they do not conduct electricity in their solid state, as the ions are locked in their positions and cannot move freely.

(d) Network of covalent bonds

: A solid with a network of covalent bonds is a covalent (or network) solid. In covalent solids, atoms are bonded to each other by strong covalent bonds in a large network or lattice. These solids are usually very hard, have high melting points, and are poor electrical conductors due to the absence of free-moving charged particles. Examples of covalent solids are diamond and quartz.

Key concepts

Metallic Solids

Understanding metallic solids unlocks the properties of many common materials from copper wires to gold jewelry. Metallic solids are characterized by a unique bonding called metallic bonding. Imagine countless metal atoms packed tightly, but instead of sharing or transferring electrons specifically, these electrons are free to move throughout the structure—a phenomenon known as the sea of electrons.

This mobility is crucial; it accounts for the high electrical and thermal conductivity of metals, allowing them to carry current or heat with ease. Metals also have a shiny appearance and are malleable, which means they can be hammered into shapes without breaking. These characteristics arise from the capacity of electrons to absorb and re-emit light, and the ability of the electron sea to flow, maintaining cohesion even when the metal is reshaped.

Molecular Solids

Now, let's ponder the gentler side of crystalline solids: molecular solids. Common examples include ice, dry ice (solid CO₂), and sugar. These solids are formed from molecules held together by intermolecular forces rather than bonds between atoms.

Weak Links Between Molecules

Molecular solids typically have lower melting and boiling points because these 'weak links', like Van der Waals forces, hydrogen bonds, and dipole-dipole interactions, require less energy to separate. Hence, they often exhibit a characteristic softness, and because molecules in these solids are not free to move, they are poor conductors of electricity. The various kinds of intermolecular forces dictate the physical properties we observe, like the slippery feel of ice (solid water) due to hydrogen bonds.

Ionic Solids

Diving into the realm of ionic solids, we encounter substances that are pivotal in chemistry and materials science. Examples include table salt (NaCl) and many minerals. Ionic solids form when oppositely charged ions attract each other and arrange into a repeating 3D lattice.

Strong Forceful Lattices

Because of the significant electrostatic forces at play between ions, these solids tend to have high melting points. In their solid state, they're rigid and non-conductive, as ions can't move to carry an electric charge. However, when dissolved in water or melted into a liquid, ionic compounds can conduct electricity due to the free movement of ions. This dual nature is pivotal in a multitude of applications, from the salinity of oceans to the functioning of batteries.

Covalent Solids

Last but not least are covalent solids, also called network solids, which embody robustness in the crystalline realm. Think of diamonds, quartz, and silicon carbide—materials known for their impressive durability.

Bound by Strength

Each atom in a covalent solid is connected to its neighbors by covalent bonds, forming a continuous network that extends throughout the entire material. This gives rise to their incredibly high melting points and hardness. Since there are no charged particles that can freely move about, covalent solids are typically poor conductors of electricity. The strength and stability of covalent solids make them ideal for a variety of applications, from cutting tools (using diamonds) to electronics (employing silicon).