Question

How does the Lewis model for covalent bonding account for the rela- tively low melting and boiling points of molecular compounds (compared to ionic compounds)?

Short answer

The Lewis model accounts for low melting and boiling points of molecular compounds by showing that they are comprised of discrete molecules held together by relatively weak intermolecular forces, as opposed to the strong ionic bonds in ionic compounds.

Step-by-step solution

Understanding Lewis Model

The Lewis model for covalent bonding explains how atoms share pairs of electrons to achieve a stable electron configuration, typically represented by Lewis dot structures. Covalent bonds form between non-metal atoms with similar electronegativities. In this model, molecules are formed as discrete units.

Understanding Physical State

Molecular compounds exist as separate, discrete molecules. The forces between these molecules are intermolecular forces (such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds), which are generally much weaker than the ionic or covalent bonds within the molecules.

Relating Intermolecular Forces to Melting and Boiling Points

The melting and boiling points of molecular compounds tend to be lower than those of ionic compounds because the intermolecular forces that hold the molecules together in solid and liquid states are much weaker than the ionic bonds in ionic compounds. Therefore, less energy (heat) is required to overcome these forces during phase transitions, such as melting and boiling.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractive forces that occur between separate molecules, which are essential in understanding the physical properties of molecular compounds. Unlike the strong intramolecular forces that hold atoms together within a molecule, such as covalent bonds, intermolecular forces are comparatively weak.

There are several types of intermolecular forces, including London dispersion forces, present in all molecular compounds; dipole-dipole interactions, found in molecules with a permanent dipole moment; and hydrogen bonding, a special case of dipole-dipole interactions occurring in molecules where a hydrogen atom is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine.

The strength of these forces plays a critical role in determining the melting and boiling points of substances. For example, hydrogen bonds are relatively stronger than dipole-dipole and London dispersion forces, resulting in higher melting and boiling points for substances that exhibit hydrogen bonding.

Melting and Boiling Points

The melting point of a substance is the temperature at which it transitions from a solid to a liquid, while the boiling point is where it changes from a liquid to a gas. These phase transitions involve overcoming intermolecular forces to allow molecules to move more freely.

For molecular compounds with weak intermolecular forces, like those held together by London dispersion forces, the energy required to overcome these forces is relatively low. This explains why substances such as noble gases or nonpolar organic molecules have low melting and boiling points. In contrast, molecules with strong intermolecular forces, such as water with its hydrogen bonds, require more energy to change phases, leading to higher melting and boiling points.

Understanding the relationship between intermolecular forces and melting/boiling points is crucial in various applications, from the design of refrigerants with low boiling points to the selection of lubricants with specific melting points.

Molecular Compounds

Molecular compounds consist of discrete molecules held together by covalent bonds, whereby atoms share electron pairs to achieve stable electron configurations. This concept is part of the Lewis model for covalent bonding, where Lewis dot structures can visually represent how these electron pairs are shared between atoms.

These molecules can be simple, like oxygen gas (o2), or complex, such as large organic molecules. The intermolecular forces between these molecules are not nearly as strong as the covalent bonds that hold the atoms together within a molecule. Because of this, molecular compounds typically have lower melting and boiling points than ionic compounds, where ions are held together by the much stronger ionic bonds in a crystalline lattice structure.

Furthermore, the physical properties of molecular compounds, including their state, solubility, and volatility, are largely dependent on the nature of intermolecular forces acting between the molecules, making an understanding of these forces key to predicting and explaining these properties.