Question

Classify each of the following statements as true or false: $$ \begin{array}{l}{\text { (a) Although both molecular solids and covalent- network }} \\ {\text { solids have covalent bonds, the melting points of molec- }} \\ {\text { ular solids are much lower because their covalent bonds }} \\ {\text { are much weaker. }} \\ {\text { (b) Other factors being equal, highly symmetric molecules }} \\ {\text { tend to form solids with highly symmetric molecules }} \\ {\text { asymmetrically shaped molecules. }}\end{array} $$

Short answer

(a) Partially True: Molecular solids have lower melting points due to weaker intermolecular forces, not weaker covalent bonds. (b) True: Highly symmetric molecules tend to form solids with other highly symmetric molecules, providing a more efficient packing arrangement.

Step-by-step solution

Analyzing statement (a)

To determine if statement (a) is true or false, let's break it down into two parts: 1. Molecular solids and covalent-network solids both have covalent bonds. 2. The melting points of molecular solids are much lower because their covalent bonds are much weaker. In both molecular solids and covalent-network solids, the atoms are held together by covalent bonds. However, the key difference is that molecular solids have a lattice of individual molecules held together by intermolecular forces (such as London dispersion forces, dipole-dipole interactions, and hydrogen bonds), while covalent-network solids have a continuous network of covalently bonded atoms. The melting points of molecular solids are influenced by the strength of the intermolecular forces, not the strength of the covalent bonds within the individual molecules. Since the intermolecular forces are generally weaker than the covalent bonds within the network of covalent-network solids, molecular solids have lower melting points. Thus, statement (a) is partially true. Molecular solids do have lower melting points, but not because their covalent bonds are weaker. It is due to the weaker intermolecular forces holding the individual molecules together.

Analyzing statement (b)

To determine if statement (b) is true or false, let's break it down: 1. Highly symmetric molecules tend to form solids with highly symmetric molecules. 2. Asymmetrically shaped molecules. The statement (b) seems to be incomplete and unclear. However, one possible interpretation is: Highly symmetric molecules tend to form solids with other highly symmetric molecules, instead of asymmetrically shaped molecules. Nevertheless, the tendency for molecules to form solids depends on factors such as the type and strength of their intermolecular forces, as well as their packing efficiency. While symmetry may not be the primary deciding factor, it can play a role in the packing efficiency of molecules in a solid. Molecules with high symmetry, such as spheres, usually pack more efficiently than asymmetric ones. Consequently, highly symmetric molecules are more likely to form solids with other highly symmetric molecules, providing a more efficient packing arrangement. Therefore, based on this interpretation, statement (b) can be considered true.

Final answers

Based on the analysis: (a) Partially True: Molecular solids do have lower melting points, but not because their covalent bonds are weaker. It is due to the weaker intermolecular forces holding the individual molecules together. (b) True: Highly symmetric molecules tend to form solids with other highly symmetric molecules, providing a more efficient packing arrangement.

Key concepts

Intermolecular Forces

Intermolecular forces are the interactions that occur between molecules, which can greatly influence the physical properties of a substance, including its phase, melting and boiling points, and solubility. Unlike covalent bonds that hold atoms together within a molecule, intermolecular forces are weaker and act between separate molecules.

There are several types of intermolecular forces, each differing in strength and origin:

• London dispersion forces - Usually the weakest, present in all molecules, resulting from temporary dipoles.
• Dipole-dipole interactions - These occur between molecules with permanent dipole moments.
• Hydrogen bonds - A special type of dipole-dipole interaction, occurring when hydrogen is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine.

The strength of these forces determines how easily the molecules can move past each other, thus affecting the melting point of a molecular solid. The weaker the intermolecular force, the lower the melting point, since less energy is required to overcome these forces during the phase change.

Covalent Bonds

Covalent bonds are the strong chemical bonds formed when two atoms share pairs of electrons. This type of bonding occurs between atoms with similar electronegativities, where neither atom has a strong enough pull to completely remove electrons from the other. As a result, these atoms share electrons to achieve a full valence shell, leading to inherently stable structures.

In covalent-network solids, such as diamond or silica, atoms are linked by a vast network of covalent bonds, creating a continuous, three-dimensional structure. This results in solids that are extremely hard and have high melting points, as a large amount of energy is required to disrupt the network of covalent bonds throughout the entire material.

Melting Points of Solids

The melting point of a solid is the temperature at which it transitions from a solid to a liquid. This phase change occurs when the molecules or atoms within the solid have enough kinetic energy to overcome the forces holding them in place. The nature of these forces - whether they be intermolecular forces in molecular solids or covalent bonds in covalent-network solids - defines how much energy, in the form of heat, is needed for the solid to melt.

Molecular solids, with their less robust intermolecular forces, tend to have lower melting points compared to covalent-network solids. In contrast, covalent-network solids require significant amounts of energy to break down their extensive covalent bonding networks, resulting in remarkably high melting points.

Molecular Symmetry in Solids

Molecular symmetry refers to the balanced and proportionate arrangement of atoms within a molecule. A symmetrical molecule will have evenly distributed electron clouds and balanced physical properties, while an asymmetrical molecule will have an uneven distribution.

When it comes to the formation of solids, high molecular symmetry often leads to more efficient packing in the solid state. Symmetric molecules like spheres or cubes can stack neatly and maximize the number of interactions between molecules, resulting in greater stability of the solid. Conversely, asymmetric molecules may not pack as efficiently, potentially leading to lower densities and possibly affecting other physical properties, such as melting points. Thus, molecular symmetry can influence how a substance forms a solid and how its molecules are arranged within that solid.