Problem 72 - Chemistry: The Central Science

Question

Indicate the type of crystal (molecular, metallic, covalent-network, or ionic) each of the following would form upon solidification: (a) \(\mathrm{CaCO}_{3}\), (b) \(\mathrm{Pt}\), (c) \(\mathrm{ZrO}_{2}\) (melting point, \(2677^{\circ} \mathrm{C}\) ), (d) table sugar \(\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)\) (e) benzene, (f) \(\mathrm{I}_{2}\).

Step-by-Step Solution

Answer

(a) \(\mathrm{CaCO}_{3}\): Ionic crystal, (b) \(\mathrm{Pt}\): Metallic crystal, (c) \(\mathrm{ZrO}_{2}\): Ionic crystal, (d) Table sugar \(\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)\): Molecular crystal, (e) Benzene: Molecular crystal, (f) \(\mathrm{I}_{2}\): Molecular crystal.

Step 1: Understand the crystal types

Before identifying the crystal types of each substance, we must understand the characteristics of each type: 1. Molecular crystals: Formed by nonmetal elements or molecules held together by weak intermolecular forces such as London dispersion forces, dipole-dipole interactions, or hydrogen bonding. 2. Metallic crystals: Formed by metal elements held together by a loose bonding arrangement where metal ions are surrounded by a sea of valence electrons. 3. Covalent-network crystals: Formed by nonmetal elements held together by strong covalent bonds throughout the entire crystal lattice. 4. Ionic crystals: Formed by the combination of metal cations and nonmetal anions held together by strong electrostatic forces (ionic bonds). Step 2: Classify each substance based on its chemical formula or properties

Step 2: Identify the crystal type for each substance

(a) \(\mathrm{CaCO}_{3}\): Calcium carbonate is composed of a metal cation (Ca\(^{2+}\)) and a polyatomic anion (CO\(_{3}^{2-}\)), which are held together by strong ionic bonds. Thus, it forms an ionic crystal. (b) \(\mathrm{Pt}\): Platinum is a metal element. The atoms in metallic crystals are held together by a sea of valence electrons. Hence, it forms a metallic crystal. (c) \(\mathrm{ZrO}_{2}\): Zirconium dioxide is a compound formed from a metal (Zr) and a nonmetal (O), which together form ionic bonds (Zr\(^{4+}\) and O\(^{2-}\)). Its high melting point also indicates the presence of ionic bonds. Therefore, it forms an ionic crystal. (d) Table sugar \(\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)\): Table sugar is a compound formed by nonmetal elements (C, H, and O) and is a large molecule. The interactions between sugar molecules are due to weak intermolecular forces; hence, it forms a molecular crystal. (e) Benzene: Benzene is an organic compound with nonmetal elements (C and H) and is a molecular substance. The intermolecular forces between benzene molecules are weak London dispersion forces and weak dipole-dipole interactions. Thus, it forms a molecular crystal. (f) \(\mathrm{I}_{2}\): Iodine is a nonmetal element that forms diatomic molecules, held together by weak London dispersion forces. Therefore, I\(_2\) forms a molecular crystal.

Key Concepts

Molecular Crystals

Imagine a gentle dance where partners barely touch each other; this is akin to the way molecules are held together in molecular crystals. In these crystals, the individual molecules are the dancers, and they're connected by relatively weak intermolecular forces such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds. These forces are much weaker than the bonds in other types of crystals, which gives molecular crystals their characteristic low melting points.

For example, table sugar and iodine are typical molecular crystals. The sugar molecules (C_{12}H_{22}O_{11}) interact through delicate van der Waals forces that can be easily broken apart—hence, why sugar melts in your tea! Similarly, iodine (I_2) crystals are composed of diatomic molecules held together by feeble London dispersion forces. When studying molecular crystals, it's crucial to appreciate the fragility and subtlety of these interactions.

Metallic Crystals

The lustrous shine of metals is not their only unique feature; metallic crystals exhibit a special type of bonding where valence electrons wander freely throughout the structure, like a communal sea. This 'sea of electrons' surrounding metal cations gives metallic crystals their characteristic properties, such as high conductivity, malleability, and ductility.

Take platinum (Pt) as an example. Its atoms are embedded in a sea of delocalized electrons, reminiscent of a ballroom filled with dancers gliding smoothly over the floor, while their shared energy sustains the cohesion and flow of the dance. This 'metallic bond' is what allows metals to conduct electricity and heat so well and to deform without breaking.

Covalent-Network Crystals

If molecular crystals are like a gentle dance, covalent-network crystals are more akin to a fortress. In these crystals, atoms are linked together in a robust, continuous network by strong covalent bonds. This arrangement forms a rigid, three-dimensional lattice that extends throughout the entire crystal, leading to extremely high melting points and great hardness.

One of the most famous examples is diamond, with carbon atoms bonded to each other in a three-dimensional lattice. These crystals are known for their impressive structural integrity and are often used in applications that require materials with high durability, such as cutting tools or abrasives. Remember that these bonds are mighty, and breaking a covalent-network crystal is like trying to breach a fortified castle wall—it's no easy feat!

Ionic Crystals

Just like magnets that stick together, ionic crystals are composed of positively and negatively charged ions held together by strong electrostatic forces—the ionic bonds. These forces give ionic crystals their solidity and high melting points. Consider the kitchen staple, table salt (sodium chloride); it's a perfect example of an ionic crystal, with each sodium ion (Na^+) and chloride ion (Cl^-) alternating in a repeating lattice.

When examining ionic crystals such as calcium carbonate (CaCO_3) or zirconium dioxide (ZrO_2), it's impressive to note how the ionic bonds form a rigid structure that can only be broken with significant heat or energy. This stability is part of what makes ionic compounds so durable, and these strong ionic interactions are key to their high melting and boiling points.