Question

What type of solid will each of the following substances form? a. \(\mathrm{CO}_{2}\) b. \(\mathrm{SiO}_{2}\) c. \(\mathrm{Si}\) d. \(\mathrm{CH}_{4}\) e. \(\mathbf{R} \mathbf{u}\) f. \(\mathrm{I}_{2}\)

Short answer

a. CO₂ forms a molecular solid due to weak van der Waals forces between its linear nonpolar molecules. b. SiO₂ forms a covalent solid due to its three-dimensional network structure of tetrahedrally arranged silicon atoms bonded to oxygen atoms. c. Si forms a covalent solid due to its crystal lattice structure with tetrahedrally arranged covalently bonded silicon atoms. d. CH₄ forms a molecular solid due to weak van der Waals forces between its tetrahedral, nonpolar methane molecules. e. Ru forms a metallic solid due to its lattice structure of positively charged atoms with free electrons. f. I₂ forms a molecular solid due to van der Waals forces holding together its diatomic iodine molecules.

Step-by-step solution

a. CO₂

Carbon dioxide (CO₂) is a linear, nonpolar molecule consisting of two covalent double bonds between the carbon atom and each of the two oxygen atoms. These molecules will form a molecular solid in which weak van der Waals forces hold the individual CO₂ molecules together.

b. SiO₂

Silicon dioxide (SiO₂), also known as silica, has a three-dimensional network structure in which each silicon atom is covalently bonded to four oxygen atoms in a tetrahedral arrangement. This extensive covalent network results in SiO₂ forming a covalent solid.

c. Si

Silicon (Si) is a non-metal, but it has metallic properties due to its crystal structure and its position in the periodic table. Silicon atoms form a crystal lattice with each atom covalently bonded to four neighbouring silicon atoms in a tetrahedral arrangement. So, silicon forms a covalent solid.

d. CH₄

Methane (CH₄) is a tetrahedral, nonpolar molecule, with covalent single bonds between the carbon atom and each of the four hydrogen atoms. Similar to CO₂, these molecules will form a molecular solid in which weak van der Waals forces hold the individual CH₄ molecules together.

e. Ru

Ruthenium (Ru) is a metal element under the platinum group of elements in the periodic table. As a metal, Ru consists of a lattice of positively charged atoms immersed in a sea of free electrons, which allows them to conduct electricity and contribute to metallic bonding. Ruthenium forms a metallic solid.

f. I₂

Elemental iodine (I₂) exists as diatomic molecules joined together by a single covalent bond between two iodine atoms. These I₂ molecules interact with each other through van der Waals forces, forming a molecular solid.

Key concepts

Molecular Solids

Molecular solids are a fascinating type of solid where the structure is made up of molecules held together by relatively weak forces. These forces are usually van der Waals forces, which are much weaker than the covalent or ionic bonds within the molecules themselves.
Factors contributing to the formation of molecular solids include the nature of the molecules involved and the type of intermolecular forces that dominate. Common examples of molecular solids include solid forms of gases like carbon dioxide (CO₂) and methane (CH₄).

• These solids have low melting points due to the weak forces holding their structures together.
• They are generally soft and may exist in gaseous forms under ordinary conditions.
• Molecular solids do not conduct electricity as they do not have free ions or electrons.

They make up some everyday substances, such as ice and solid iodine, which all highlight the diversity within molecular solids.

Covalent Solids

Covalent solids, also known as network solids, are characterized by an extensive network of covalent bonds. This type of solid includes materials like silicon dioxide (SiO₂) and pure silicon (Si).
These solids are incredibly sturdy because each atom is bonded to several neighbors, forming a fully interconnected lattice structure.

• They usually have high melting points due to the strength of the covalent bonds in their structures.
• Covalent solids are typically hard and brittle, making them suitable for various industrial applications.
• They are poor conductors of electricity since they have no free charged particles.

Understanding covalent solids unlocks insights into materials science and the characteristics that make substances like diamonds exceptionally hard.

Metallic Solids

Metallic solids are formed by metallic elements where atoms are arranged in a sea of delocalized electrons. This electron cloud around positively charged metal ions enables interesting properties.
Ruthenium (Ru) is a stellar example of a metallic solid, showcasing the distinctive traits of metallic bonding.

• The sea of electrons provides metallic solids with good conductivity, as the electrons can move freely.
• The layers of atoms can slide over each other, which accounts for the malleability and ductility of metals.
• Metallic solids also often have a high melting point and are usually lustrous.

The unique structure of metallic solids is crucial for applications that require flexibility and strength, from electrical wiring to structural frameworks.

van der Waals Forces

Van der Waals forces are weak attractions that occur between molecules. These forces are not as strong as covalent, ionic, or metallic bonds, yet they play a crucial role in the formation of molecular solids.
Van der Waals forces include a variety of interactions such as dipole-dipole interactions, hydrogen bonding, and dispersion forces.

• They have a particularly significant impact on substances that have low melting and boiling points.
• Notably influential in liquids and gases, they become key when these states condense into solids.
• Molecules like I₂, which exist as diatomic entities, utilize these forces in forming molecular solids.

van der Waals forces also offer explanations for biologically significant events, such as protein folding, making understanding these forces fundamental to chemistry.