Question

The melting points of the oxides of the third-period elements are given in parentheses: \(\mathrm{Na}_{2} \mathrm{O}\left(1275^{\circ} \mathrm{C}\right)\) \(\mathrm{MgO}\left(2800^{\circ} \mathrm{C}\right), \mathrm{Al}_{2} \mathrm{O}_{3}\left(2045^{\circ} \mathrm{C}\right), \mathrm{SiO}_{2}\left(1610^{\circ} \mathrm{C}\right), \mathrm{P}_{4} \mathrm{O}_{10}\) \(\left(580^{\circ} \mathrm{C}\right), \mathrm{SO}_{3}\left(16.8^{\circ} \mathrm{C}\right), \mathrm{Cl}_{2} \mathrm{O}_{7}\left(-91.5^{\circ} \mathrm{C}\right) .\) Classify these solids in terms of crystal types.

Short answer

\(\text{Na}_{2}\text{O}, \text{MgO}, \text{Al}_{2}\text{O}_{3}\) are ionic; \(\text{SiO}_{2}\) is covalent (network); \(\text{P}_{4}\text{O}_{10}\), \(\text{SO}_{3}\), \(\text{Cl}_{2}\text{O}_{7}\) are molecular.

Step-by-step solution

Identify Crystal Types

Determine the different types of crystal structures present. The four main types of crystalline solids are ionic, metallic, covalent (network), and molecular.

Classify Each Oxide

Assign a crystal type to each given oxide based on its properties: - \(\text{Na}_{2}\text{O}\) and \(\text{MgO}\) are ionic compounds, known for high melting points.- \(\text{Al}_{2}\text{O}_{3}\) is ionic with some covalent character, also with a high melting point.- \(\text{SiO}_{2}\) is a covalent (network) solid, also known for a high melting point due to strong bonds.- \(\text{P}_{4}\text{O}_{10}\), \(\text{SO}_{3}\), and \(\text{Cl}_{2}\text{O}_{7}\) are molecular compounds, having relatively lower melting points.

Summarize the Classification

List the compounds based on their crystalline structure:- **Ionic Crystals:** \(\text{Na}_{2}\text{O}, \text{MgO}, \text{Al}_{2}\text{O}_{3}\) (though \(\text{Al}_{2}\text{O}_{3}\) has substantial covalent character, it is generally classified with ionic compounds initially).- **Covalent (Network) Crystals:** \(\text{SiO}_{2}\).- **Molecular Crystals:** \(\text{P}_{4}\text{O}_{10}, \text{SO}_{3}, \text{Cl}_{2}\text{O}_{7}\).

Key concepts

Ionic Crystals

Ionic crystals are a fascinating type of solid structure that arises from the electrostatic attraction between positively and negatively charged ions. In our current focus, compounds like Na\(_2\)O and MgO serve as prime examples of ionic crystals. These materials typically have high melting points, making them quite stable and robust. The strong bond formed from the ionic interactions requires a large amount of energy to break.

One of the key features of ionic crystals is their formation through the transfer of electrons. The metal atoms donate electrons, forming positive ions, while the non-metal atoms accept electrons, becoming negative ions. This electron transfer is what results in the resultant electrostatic attraction. Ionic compounds are often characterized by their high density and hardness.

• High melting points: Due to strong ionic bonds
• Solid at room temperature: Most ionic compounds are solid because of lattice energy
• Conductive in liquid form: Although solid ionic crystals do not conduct electricity, their liquid form or when dissolved in water becomes conductive

Covalent Network Crystals

Covalent network crystals, unlike ionic crystals, involve atoms held together by covalent bonds. In a covalent network, each atom is bonded to its neighbors in a fixed, 3D pattern throughout the entire piece of the material. Silicon dioxide (SiO\(_2\)) is a classic example of this type of solid structure. These materials also generally exhibit high melting points due to the strong covalent bonds holding the network together.

In covalent network solids, atoms share electrons and form extensive 3D networks or planes. This continuous network makes them very hard and durable. Such properties are why materials like quartz and diamond are categorized as covalent network solids. Here are some common traits:

• High hardness: Due to strong, continuous covalent bonding
• Poor electrical conductivity: The lack of free ions or electrons prevents electrical flow.
• High melting points: Strong covalent bonds take substantial energy to break
  
  

Despite their hardness, these materials are often brittle. This is because when a force is applied, the bonds break rather than bending.

Molecular Crystals

Molecular crystals are composed of molecules held together by van der Waals forces, hydrogen bonds, or dipole-dipole interactions. Unlike the stronger ionic and covalent bonds we see in other crystal types, these forces are much weaker, giving molecular crystals distinctive traits such as low melting points and softness.

In the case of oxides such as P\(_4\)O\(_{10}\), SO\(_3\), and Cl\(_2\)O\(_7\), the weak intermolecular forces hold the molecules in the solid form.

Molecular crystals are quite common in organic compounds and some inorganic ones, characterized by these properties:

• Low melting points: Due to weak intermolecular forces
• Soft texture: They do not have the firm lattices found in ionic or covalent network crystals
• Non-conductive: Electrons are generally localized within the molecules, hindering conductivity

Despite their fragile nature, molecular crystals find applications where low strength and low thermal resistance are required, such as in organic compounds and biologically active molecules.