Problem 14 - Chemistry: The Central Science in SI Units, Global Edition

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; \((\mathbf{d})\) network of covalent bonds.

Step-by-Step Solution

Answer

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

Step 1: (a) High mobility of electrons throughout the solid

A solid with high mobility of electrons throughout it is characterized as a metallic solid. In metallic solids, the valence electrons are shared among all the atoms, and this allows for high conductivity, ductility, and favorable electrical properties.

Step 2: (b) Softness, relatively low melting point

Softness and a low melting point are characteristics of molecular solids. In molecular solids, the atoms are held together by weak van der Waals forces, hydrogen bonds, or dipole-dipole interactions, which facilitate a relatively easier separation of the particles compared to other types of crystalline solids.

Step 3: (c) High melting point and poor electrical conductivity

A solid with a high melting point and poor electrical conductivity is an ionic solid. Ionic solids are composed of positive and negative ions arranged in an ordered fashion and held together by strong ionic bonds. The high melting point is due to the strong electrostatic forces between the ions, but the poor electrical conductivity is a result of immobile ions in the solid state.

Step 4: (d) Network of covalent bonds

A solid that has a network of covalent bonds is a covalent network solid. In covalent network solids, the atoms are interconnected by a continuous network of covalent bonds, making them very strong and rigid. Some examples include diamond (carbon atoms connected by covalent bonds) and quartz (silicon dioxide).

Key Concepts

Metallic Solids

Metallic solids are distinguished by their ability to conduct electricity efficiently. This is due to the presence of free-flowing valence electrons, often referred to as an "electron sea." These electrons are not bound to any specific atom and can move throughout the solid, which results in high electrical conductivity. The mobility of these electrons also lends metallic solids their ductility and malleability, allowing them to be shaped without breaking. Furthermore, the strong metallic bonds within these solids contribute to their lustrous appearance and overall robustness.

Molecular Solids

Molecular solids are generally soft and possess relatively low melting points. This can be attributed to the weak forces that hold the molecules together within the solid, such as van der Waals forces, dipole-dipole interactions, and hydrogen bonds. These weak intermolecular forces make molecular solids easy to separate, causing them to melt at lower temperatures. A good example of a molecular solid is ice, where water molecules are held together by hydrogen bonds.

Ionic Solids

Ionic solids consist of positive and negative ions arranged in a regular lattice. The strong electrostatic interactions between these oppositely charged ions result in high melting points. However, in solid form, ionic solids are poor conductors of electricity. This is because ions are locked in place within the crystal lattice and cannot flow freely. Once melted or dissolved in water, ionic solids can conduct electricity because the ions are free to move. Examples include table salt (NaCl) and potassium bromide (KBr).

Covalent Network Solids

Covalent network solids are some of the toughest substances on Earth. Their atoms are bonded covalently in an extended network, endowing them with high melting points and extreme hardness. They lack discrete molecules, as every atom is connected to others via strong covalent bonds. This network feature means that breaking the structure requires considerable energy, making these solids rigid and non-conductive. Classic examples include diamond, made of carbon atoms, and quartz, comprised of silicon and oxygen atoms.