Problem 19 - Chemistry: The Central Science

Question

(a) What is meant by the term polarizability? (b) Which of the following atoms would you expect to be most polarizable: N, P, As, Sb? Explain. (c) List the following molecules in order of increasing polarizability: \(\mathrm{GeCl}_{4}, \mathrm{CH}_{4}, \mathrm{SiCl}_{4}, \mathrm{SiH}_{4}\), and \(\mathrm{GeBr}_{4}\). (d) Predict the order of boiling points of the substances in part (c).

Step-by-Step Solution

Answer

(a) Polarizability refers to the ability of an atom or molecule to be distorted by an electric field, leading to the redistribution of electron density within the atom or molecule. (b) Sb is the most polarizable among N, P, As, and Sb, as it is the largest and has the most electrons. (c) The order of increasing polarizability is: \(\mathrm{CH}_{4} < \mathrm{SiH}_{4} < \mathrm{GeCl}_{4} < \mathrm{SiCl}_{4} < \mathrm{GeBr}_{4}\). (d) The order of boiling points, which are influenced by polarizability, is: \(\mathrm{CH}_{4} < \mathrm{SiH}_{4} < \mathrm{GeCl}_{4} < \mathrm{SiCl}_{4} < \mathrm{GeBr}_{4}\).

Step 1: (a) Definition of Polarizability

Polarizability refers to the ability of an atom or molecule to be distorted by an electric field, leading to the redistribution of electron density within the atom or molecule. Highly polarizable atoms or molecules have electrons that can be more easily displaced.

Step 2: (b) Most Polarizable Atom

To determine the most polarizable atom among N, P, As, and Sb, we should consider the size of the atoms and their electron distribution. Larger atoms with more electrons are generally more polarizable. In this case, antsimony (Sb) is the largest and has the most electrons, so we expect it to be the most polarizable.

Step 3: (c) Ordering Molecules by Polarizability

To order the molecules by polarizability, we should consider both the sizes of the central atoms and the types of atoms bonded to them. More polarizable molecules will have larger central atoms and more polarizable bonded atoms. The order of increasing polarizability will be: \(\mathrm{CH}_{4} < \mathrm{SiH}_{4} < \mathrm{GeCl}_{4} < \mathrm{SiCl}_{4} < \mathrm{GeBr}_{4}\).

Step 4: (d) Predict Boiling Points

Boiling points are influenced by intermolecular forces, which are stronger for more polarizable molecules. Based on the polarizability order, we can predict the order of boiling points as follows: \(\mathrm{CH}_{4} < \mathrm{SiH}_{4} < \mathrm{GeCl}_{4} < \mathrm{SiCl}_{4} < \mathrm{GeBr}_{4}\). The boiling point increases with increasing polarizability because of stronger London dispersion forces.

Key Concepts

Electron Density

Electron density refers to the distribution of electrons around an atom or molecule. This distribution determines many chemical properties, including reactivity and polarity.
In a simple sense, it describes where electrons are likely to be found at any given time.

• The regions in a molecule where electron density is high are more likely to participate in chemical reactions.
• Areas with higher electron density may lead to stronger intermolecular attractions, such as London dispersion forces.

Overall, electron density is fundamental to understanding chemical behavior and intermolecular interactions.

Intermolecular Forces

Intermolecular forces are the attraction forces between molecules, holding them together in different states of matter. These forces play a crucial role in determining the physical properties of substances, such as boiling and melting points.
These forces can come from various origins:

• **Dipole-dipole interactions** are found between polar molecules with permanent dipoles.
• **Hydrogen bonds** are a type of strong dipole-dipole interaction that occurs when hydrogen is bonded to a very electronegative atom, like oxygen or nitrogen.
• **London dispersion forces** are a type of force present in all molecules due to temporary shifts in electron density.

Although weaker than chemical bonds, intermolecular forces are essential for the state and behavior of substances.

London Dispersion Forces

London dispersion forces, also known as dispersion forces or induced dipole-induced dipole attractions, are a type of weak intermolecular force. These forces occur between all molecules, whether polar or nonpolar, due to temporary fluctuations in electron density.

• These temporary shifts create a momentary dipole that induces a dipole in a neighboring atom or molecule.
• The strength of London dispersion forces increases with the number of electrons present, meaning larger atoms or molecules are usually more affected.

Therefore, a larger electron cloud, which is more polarizable, will result in stronger London dispersion forces, affecting properties such as boiling point and solubility.