Problem 18 - Chemistry: The Central Science

Question

True or false: (a) The more polarizable the molecules, the stronger the dispersion forces between them. (b) The boiling points of the noble gases decrease as you go down the column in the periodic table. (c) In general, the smaller the molecule, the stronger the dispersion forces. (d) All other factors being the same, dispersion forces between molecules increase with the number of electrons in the molecules.

Step-by-Step Solution

Answer

(a) True: The more polarizable the molecules, the stronger the dispersion forces between them. (b) False: The boiling points of the noble gases increase as you go down the column in the periodic table. (c) False: In general, the smaller the molecule, the weaker the dispersion forces. (d) True: Dispersion forces between molecules increase with the number of electrons in the molecules, if all other factors remain constant.

Step 1: Statement (a)

The dispersion forces, also known as London dispersion forces or van der Waals forces, are intermolecular forces that arise from the temporary fluctuations in electron density around molecules. The more polarizable a molecule, the more capable it is of undergoing those temporary fluctuations, leading to stronger interactions with other molecules. Hence, the statement is True.

Step 2: Statement (b)

The boiling points of noble gases are primarily dependent on dispersion forces as they are inert and do not form chemical bonds. As we move down the column in the periodic table, the atomic size and the number of electrons increase, hence the dispersion forces become stronger. This results in an increase in boiling points as you go down the group. Therefore, the statement is False.

Step 3: Statement (c)

In general, larger molecules have more electrons and a larger electron cloud, increasing the polarizability and dispersion forces. So, if the molecule is smaller, its dispersion forces will be weaker in comparison. Consequently, the statement is False.

Step 4: Statement (d)

Dispersion forces arise from temporary fluctuations in electron density around the molecules. The more electrons a molecule has, the greater the chance of temporary fluctuations, which results in stronger dispersion forces. If all other factors are constant, the dispersion forces between molecules will increase with the number of electrons in the molecules. Thus, the statement is True.

Key Concepts

Polarizability

Polarizability refers to the ease with which the electron cloud of a molecule can be distorted. This distortion happens due to an external electric field or the presence of nearby ions or molecules.
When a molecule is highly polarizable, it means that its electron cloud can move more freely, allowing temporary fluctuations in electron density.
These fluctuations enhance the ability of a molecule to engage in dispersion forces, which are a type of weak intermolecular interaction.

• A more polarizable molecule can create stronger temporary dipoles.
• Stronger temporary dipoles lead to stronger attraction between molecules.

For instance, larger atoms and molecules typically have more polarizable electron clouds because they have more electrons and the outer electrons are held less tightly by the nucleus.
This is why larger molecules often have stronger dispersion forces compared to smaller ones. Understanding polarizability helps explain why certain substances have different boiling and melting points.

Boiling Points

Boiling points are a measure of the temperature at which a substance changes from a liquid to a gas.
The boiling point of a substance is largely influenced by the strength of intermolecular forces within the liquid.
For noble gases, dispersion forces are the only type of intermolecular force present, due to their inert nature.

• As more electrons are added (as we move down the group), dispersion forces increase.
• This results in higher boiling points for heavier noble gases.

However, when considering other molecules, factors such as molecular size, shape, and other types of interactions can also impact boiling points.
Generally, the stronger the intermolecular forces, the higher the boiling point, as more energy is required to overcome these forces during the phase change.

Noble Gases

Noble gases are unique elements found in Group 18 of the periodic table. They are known for their lack of reactivity due to having full valence electron shells.
Even though they do not easily form bonds, they are influenced by London dispersion forces.

• The size of the noble gas atoms increases as you move down the group in the periodic table.
• The number of electrons also increases, enhancing polarizability.

As a result, despite being chemically inert, noble gases display increasing boiling points down the group.
This is due to stronger dispersion forces acting between larger atoms with more electrons.
For example, helium has a much lower boiling point compared to radon. Understanding the properties and behavior of noble gases helps to explore the fundamental principles of chemical interactions without the complexity of chemical bonds.