Problem 13 - Chemistry: The Central Science in SI Units, Global Edition

Question

What kinds of attractive forces exist between particles (atoms, molecules, or ions) in (a) molecular crystals, (b) covalent-network crystals, (c) ionic crystals, (d) and metallic crystals?

Step-by-Step Solution

Answer

In (a) molecular crystals, the attractive forces between particles are London dispersion forces, dipole-dipole forces, and hydrogen bonding. In (b) covalent-network crystals, the attractive forces are strong covalent bonds. In (c) ionic crystals, the attractive forces are strong electrostatic or ionic bonds between oppositely charged ions. In (d) metallic crystals, the attractive forces are the metallic bonds between positively charged metal ions (cations) and negatively charged free electrons, also known as the "electron cloud" or "electron sea."

Step 1: Identify the forces in molecular crystals

Molecular crystals consist of molecules held together by London dispersion forces, dipole-dipole forces, and hydrogen bonding. The type of force depends on the molecules' nature, whether they have a dipole moment or capable of forming hydrogen bonds. #b) Covalent-network Crystals#

Step 2: Identify the forces in covalent-network crystals

Covalent-network crystals are made up of atoms covalently bonded in an extended network. The attractive forces within these crystals are strong covalent bonds, which are responsible for the high melting points and hardness of covalent-network crystals (e.g., diamond and silicon dioxide). #c) Ionic Crystals#

Step 3: Identify the forces in ionic crystals

Ionic crystals consist of positively and negatively charged ions arranged in an alternating pattern. The attractive forces within these crystals are strong electrostatic or ionic bonds between oppositely charged ions (e.g., sodium chloride (NaCl)). #d) Metallic Crystals#

Step 4: Identify the forces in metallic crystals

Metallic crystals are composed of metal atoms, typically cations, that donate their valence electrons to form a "sea" of freely moving electrons surrounding the metal cations. The attractive forces within these crystals are the metallic bonds, which refer to the electrostatic attractions between positively charged metal ions (cations) and negatively charged free electrons, typically called "electron cloud" or "electron sea." This bonding type contributes to the high electrical and thermal conductivity of metals and metallic crystals (e.g., copper and iron).

Key Concepts

Molecular Crystals

Molecular crystals are structures where the building blocks are molecules rather than individual atoms or ions. These molecules are held together by various intermolecular forces, which are much weaker compared to covalent or ionic bonds. Some common intermolecular forces include:

• London dispersion forces: These are temporary, weak forces that arise due to fluctuations in the electron distribution within nonpolar molecules, leading to temporary dipoles.
• Dipole-dipole interactions: Occur when molecules with permanent dipoles align so that the positive end of one dipole interacts with the negative end of another.
• Hydrogen bonds: A specific type of strong dipole-dipole attraction that occurs when hydrogen is bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine.

Molecular crystals, such as ice or solid carbon dioxide (dry ice), exhibit properties like low melting points and are typically soft.

Covalent-Network Crystals

Covalent-network crystals are characterized by a three-dimensional array of atoms connected by strong covalent bonds. Unlike molecular crystals, where intermolecular forces hold molecules together, covalent-network crystals rely on the covalent bonds for stability.

Materials like diamond and silicon dioxide are prominent examples. In diamond, each carbon atom is covalently bonded to four other carbon atoms, creating a rigid, hardness-endowing lattice. Similarly, in silicon dioxide, silicon atoms are bonded to oxygen atoms in a three-dimensional framework.

The strength of covalent bonds accounts for the extraordinarily high melting points, hardness, and often transparency of materials formed in this way.

Ionic Crystals

Ionic crystals are formed by the electrostatic attraction between oppositely charged ions—positively charged cations and negatively charged anions. These ions are typically arranged in an alternating, repeating pattern to form a lattice structure.

• Electrostatic interactions: The ionic bonds between the ions are incredibly strong due to the significant differences in charge, making ionic crystals hard but brittle.

Common examples of ionic crystals include sodium chloride (table salt) and magnesium oxide. These crystals usually exhibit high melting and boiling points, and are often soluble in water and other polar solvents due to their charged nature.

Metallic Crystals

Metallic crystals are unique in structure and bonding compared to other solid types. In these crystals, metal atoms are arranged in a closely packed lattice, sharing a "sea" of delocalized electrons. This electron sea allows electrons to flow freely, leading to several characteristic properties:

• Metallic bonds: Refers to the attraction between these delocalized electrons and the positively charged metal ions.
• Good electrical and thermal conductivity: Due to the mobility of electrons.
• High malleability and ductility: As the atoms can slide past each other without breaking the metallic bond.

Examples of materials with metallic crystals include copper, iron, and aluminum.

Intermolecular Forces

Intermolecular forces are the attractions between molecules or atoms within a compound or mixture, and they play a crucial role in determining the physical properties of substances. These forces are generally weaker than intramolecular forces like covalent or ionic bonds.

• London dispersion forces: Present in all molecules, especially significant in nonpolar molecules.
• Dipole-dipole interactions: Occur in polar molecules with permanent dipoles.
• Hydrogen bonds: A type of dipole interaction important in specific molecules.

These forces affect properties such as boiling and melting points, solubility, and vapor pressure. Understanding these forces helps explain the behavior of different states of matter and the transitions between them.