Question

What are London dispersion forces and how do they arise in a nonpolar molecule? Are London forces typically stronger or weaker than dipole-dipole attractions between polar molecules? Are London forces stronger or weaker than covalent bonds? Explain.

Short answer

London dispersion forces are weak intermolecular forces that arise in nonpolar molecules due to temporary fluctuations in electron distribution, creating temporary dipoles that induce dipoles in neighboring molecules. These forces are typically weaker than dipole-dipole attractions between polar molecules and significantly weaker than covalent bonds.

Step-by-step solution

Explain London dispersion forces

London dispersion forces, also known as instantaneous dipole-induced dipole forces, are weak intermolecular forces that exist between all molecules, whether they are polar or nonpolar. These forces play a significant role in understanding the properties of various substances.

Describe how London dispersion forces arise in nonpolar molecules

London dispersion forces arise due to the temporary fluctuations in the electron distribution around the molecules. Despite having no permanent dipole moment, nonpolar molecules have electrons in constant motion, which can create an instantaneous dipole for a very short period. This temporary dipole induces a similar, but opposite, dipole in neighboring molecules, leading to the formation of a weak electrostatic attraction between them. This interaction, occurring between neighboring molecules, is called a London dispersion force.

Compare the strength of London forces to dipole-dipole attractions between polar molecules

London dispersion forces are typically weaker than the dipole-dipole attractions that occur between polar molecules. Dipole-dipole attractions involve the electrostatic interaction between the permanent positive and negative charges of different polar molecules, which leads to stronger intermolecular forces than the temporary dipoles in the case of London forces.

Compare the strength of London forces to covalent bonds

London forces are significantly weaker than covalent bonds. Covalent bonds involve the sharing of electrons between atoms within a molecule, resulting in strong intramolecular forces that hold the atoms together. On the other hand, London forces are weak intermolecular forces that exist between separate molecules due to temporary fluctuations in electron distribution. In summary, London dispersion forces are weak intermolecular forces that occur between all molecules, including nonpolar molecules, due to temporary fluctuations in electron distribution. These forces are typically weaker than both dipole-dipole attractions between polar molecules and covalent bonds within molecules.

Key concepts

Intermolecular Forces

Intermolecular forces are types of forces that occur between molecules, holding them together. Unlike intramolecular forces, which exist within a molecule, intermolecular forces act between separate molecules. These forces determine the physical properties of substances, such as boiling points, melting points, and solubility.

There are different kinds of intermolecular forces, including:

• Dispersion forces, also known as London dispersion forces, present in all molecules.
• Dipole-dipole interactions, which occur between polar molecules.
• Hydrogen bonding, a strong type of dipole-dipole attraction involving hydrogen.

It's important to note that while London dispersion forces are generally weaker than dipole-dipole interactions, they are present in every type of molecule, contributing to the molecule's overall intermolecular interactions.

Nonpolar Molecules

Nonpolar molecules are those that have no permanent dipole moment; that is, their electron distribution is balanced so that there is no separation of charge across the molecule. As a result, these molecules do not have charged poles like polar molecules do.

Examples of nonpolar molecules include:

• Oxygen gas ( O_2 )
• Carbon dioxide ( CO_2 )
• Hydrocarbons like methane ( CH_4 )

Even though they lack a permanent dipole, nonpolar molecules can still experience intermolecular attractions through London dispersion forces. These forces arise because of temporary changes in electron distribution, allowing nonpolar molecules to interact weakly with each other.

Electron Distribution

Electron distribution refers to how electrons are spread across a molecule. This distribution determines whether a molecule is polar or nonpolar and affects its intermolecular interactions. In nonpolar molecules, the electron distribution is uniform, meaning electrons are evenly spread out, resulting in no regions of positive or negative charge.

However, electrons are not always perfectly evenly distributed. The constant movement of electrons can lead to short-lived imbalances that produce temporary dipoles, even in nonpolar molecules. This temporary change in electron distribution is the key factor behind London dispersion forces. By understanding electron distribution, one can better predict and explain a molecule's interactions and properties.

Instantaneous Dipole

An instantaneous dipole is a temporary dipole moment that occurs in a molecule due to a momentary uneven distribution of electrons. In any molecule, the electrons are constantly moving, and at any given moment, they may be distributed unevenly.

As a result, one end of the molecule may become slightly more negative, while the other end is slightly more positive. This instantaneous dipole can affect neighboring molecules, inducing similar dipoles in them, leading to weak attractions known as London dispersion forces.

Instantaneous dipoles are crucial to understand because they illustrate how nonpolar molecules, without a permanent dipole, can still exhibit intermolecular attractions.

Induced Dipole

An induced dipole is a temporary dipole created in a molecule when its electron cloud is distorted by the influence of a nearby instantaneous dipole. This distortion happens because the slight charge in the instantaneous dipole attracts and repels electrons in a nearby molecule, causing an imbalance.

This process is the basis for the formation of London dispersion forces. When an instantaneous dipole forms in one molecule, it can induce a dipole in an adjacent molecule, aligning them in such a way that a weak electrostatic attraction occurs. This attraction is what we observe as the London dispersion force, and it is present in every molecular interaction, albeit more noticeably in nonpolar ones due to the absence of other stronger intermolecular forces.