Problem 15 - Chemistry The Central Science

Question

Indicate the type of solid (molecular, metallic, ionic, or covalent-network) for each compound: (a) \(\mathrm{CaSO}_{4},(\mathbf{b}) \mathrm{Pd}\) , (c) \(\mathrm{Ta}_{2} \mathrm{O}_{5}\) (melting point, \(1872^{\circ} \mathrm{C} ),(\mathbf{d})\) caffeine \(\left(\mathrm{C}_{8} \mathrm{H}_{10} \mathrm{N}_{4} \mathrm{O}_{2}\right)\) (e) toluene \(\left(\mathrm{C}_{7} \mathrm{H}_{8}\right),(\mathbf{f}) \mathrm{P}_{4}\)

Step-by-Step Solution

Answer

The types of solids for the given compounds are: (a) CaSO₄ - ionic solid, (b) Pd - metallic solid, (c) Ta₂O₅ - ionic solid, (d) C₈H₁₀N₄O₂ (caffeine) - molecular solid, (e) C₇H₈ (toluene) - molecular solid, and (f) P₄ - covalent-network solid.

Step 1: a) CaSO4

Calcium sulfate (CaSO4) is a compound containing Ca²⁺, a metal cation, and SO₄²⁻, a polyatomic anion. The strong electrostatic forces between these ions make this compound an ionic solid.

Step 2: b) Pd

Palladium (Pd) is a metal, and its crystal lattice is made up of metal atoms surrounded by a "sea" of delocalized electrons. This characteristic makes Pd a metallic solid.

Step 3: c) Ta2O5

Tantalum oxide (Ta₂O₅) is a compound containing Ta, a metal atom, and O, a non-metal atom. The bond between the metal and non-metal atoms is ionic. Therefore, Ta₂O₅ is an ionic solid.

Step 4: d) C₈H₁₀N₄O₂ (Caffeine)

Caffeine is an organic molecule, with covalent bonds between the carbon, hydrogen, nitrogen, and oxygen atoms. However, the overall structure of the compound is not a continuous network. As a result, the intermolecular forces among caffeine molecules are relatively weak, such as Van der Waals forces, making caffeine a molecular solid.

Step 5: e) C₇H₈ (Toluene)

Toluene is an organic molecule, with covalent bonds between the carbon and hydrogen atoms. Like caffeine, the structure of this compound is not a continuous network, and the molecules are held together by weak intermolecular forces, such as Van der Waals forces. Thus, toluene is also a molecular solid.

Step 6: f) P₄

Phosphorus (P₄) forms a covalent-network solid, as the phosphorus atoms are connected by covalent bonds in a continuous network structure.

Key Concepts

Ionic Solid

Imagine a well-coordinated dance, where partners hold onto each other through a strong attraction, never letting go. This represents an ionic solid, where opposites truly attract and stay together. An ionic solid is formed by the electrostatic attraction between positively charged ions, known as cations, and negatively charged ions, called anions.

One classic example is \textbf{calcium sulfate} (\textbf{CaSO}\(_4\)), in which calcium ion (\textbf{Ca}^{2+}) and sulfate ion (\textbf{SO}\(_4^{2\text{-}}\)) bind together to form a rigid, durable structure. The features of ionic solids include high melting and boiling points due to the strong bonds, and they often form crystals with characteristic shapes. Usually, in solid form, ionic compounds do not conduct electricity, but when melted or dissolved in water, they become excellent conductors.

Metallic Solid

Consider a metallic solid as a communal potluck, where everyone brings something to the table. In this case, metal atoms contribute their valence electrons to a common ‘sea’ that allows them to move freely. This ‘sea of electrons’ is the reason why metallic solids like palladium (Pd) have such unique properties.

They're ductile, meaning they can be drawn into wires, and malleable, which allows them to be hammered into sheets. Furthermore, their ability to conduct heat and electricity comes from the mobility of those shared electrons. Unlike ionic or molecular solids, the structural strength and flexibility of metallic solids give them a wide range of applications in everyday life—from building materials to the fine wires inside your electronic devices.

Molecular Solid

The molecular solid is the quiet library of solids—everything's connected, but not too strongly. Molecules within a molecular solid are like books on shelves, organized and held together by relatively gentle forces. An example to consider is caffeine (C\(_8\)H\(_{10}\)N\(_4\)O\(_2\)).

It consists of individual molecules, each held together by strong covalent bonds. But between these molecules, the bonds are much weaker, generally including dipole-dipole interactions, hydrogen bonds, and London dispersion forces (Van der Waals forces). Because of these weak intermolecular forces, molecular solids tend to have low melting and boiling points. Unlike ionic solids, they often do not conduct electricity, making them insulators. The subtleties of their formation deeply impact their functions and applications.

Covalent-Network Solid

Envision a city with an intricate network of roads connecting every house and building—welcome to the world of covalent-network solids. Here, atoms like carbon, silicon, or, in our case, phosphorus (\textbf{P}\(_4\)), are all bonded together in a strong three-dimensional lattice.

Every atom is interconnected through covalent bonds, forming an extended solid that can be incredibly tough and durable. Diamonds, a form of carbon with a covalent-network, epitomize the hardness and high melting points characteristic of this solid type. These solids don't usually conduct electricity well because there are no free electrons; all of them are involved in bonding. Their robustness and resistance to heat make them invaluable in various applications, such as in cutting tools or semiconductors.