Question

Draw two HCl molecules and show how they would be attracted to each other. Show the partial charges and dipole moment vectors for both molecules, and orient the molecules properly with respect to each other.

Short answer

Two HCl molecules can be represented as \(δ^+H-Cl\:δ^-\) and \(δ^+H-Cl\:δ^-\), where H has a partial positive charge (δ+) and Cl has a partial negative charge (δ-). The dipole moment vectors (arrows) point from hydrogen to chlorine atoms. To show the dipole-dipole interaction, orient the molecules so that the oppositely charged ends are close to each other, as shown: \(δ^+H \overset{\underset{\smile}{\longrightarrow}}{|\vec{d}|}Cl \: δ^-\) and \(δ^+H \overset{\underset{\smile}{\longrightarrow}}{|\vec{d}|}Cl \: δ^-\).

Step-by-step solution

Draw the HCl molecules with their respective atoms

Draw two HCl molecules with hydrogen (H) connected to chlorine (Cl) via a single bond: \(H - Cl\) and \(H - Cl\)

Indicate the partial charges on each atom

Since chlorine is more electronegative than hydrogen, it will have a partial negative charge (δ-) and hydrogen will have a partial positive charge (δ+). The molecules will now look like: \(δ^+ H - Cl \: δ^-\) and \(δ^+ H - Cl \: δ^-\)

Represent the dipole moment vectors

A dipole moment vector indicates the direction of the dipole moment, from the positive end to the negative end. Draw an arrow from the hydrogen to the chlorine atom in each molecule, representing the dipole moment. The molecules with the dipole moment vectors will look like: \(δ^+ H - \vec{d} \to Cl \: δ^-\) and \(δ^+ H - \vec{d} \to Cl \: δ^-\)

Orient the molecules with respect to each other to show their attraction

In order to show the attraction between the two HCl molecules, orient them so that the positively charged hydrogen in one molecule is close to the negatively charged chlorine in the other molecule, and vice versa. This interaction is known as a dipole-dipole interaction. The final arrangement of the two HCl molecules with their attractions will look like: \(δ^+ H \overset{\underset{\smile}{\longrightarrow}}{|\vec{d}|}Cl \: δ^-\) and \(δ^+ H \overset{\underset{\smile}{\longrightarrow}}{|\vec{d}|}Cl \: δ^-\)

Key concepts

Electronegativity

Electronegativity is a key concept in chemistry that explains how strongly atoms attract shared electrons. Different elements have varying electronegativity values, which can significantly influence the behavior of molecules.
This property is crucial for understanding why certain atoms in a molecule carry partial charges. For instance, chlorine is more electronegative than hydrogen.
This difference means chlorine has a stronger pull on the shared electrons within the HCl molecule. When two atoms with different electronegativity form a bond, the more electronegative atom will attract the bonding electrons closer. This leads to a slight negative charge on the more electronegative atom. Meanwhile, the less electronegative atom, like hydrogen in HCl, will have a slight positive charge.
This is because the electrons are spending more time closer to the chlorine. Understanding electronegativity is crucial for predicting molecular shapes, bond strengths, and reactivity.

Partial Charges

Partial charges are the result of differences in electronegativity between two bonded atoms. They denote a separation of charge that occurs because electrons are not equally shared. In the HCl molecule, the chlorine atom, being more electronegative, gains a partial negative charge, represented as δ-. The hydrogen atom, less electronegative, gains a partial positive charge, noted as δ+.
These charges are not as intense as full ionic charges, which occur when electrons are completely transferred. Partial charges can lead to significant interactions between molecules. Such charges are the reason molecules like HCl can engage in dipole-dipole interactions. These interactions occur when the partial charge of one molecule is attracted to the opposite charge of a neighboring molecule.
Understanding partial charges helps in predicting molecular interactions and behaviors in different environments.

Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule. It is represented by a vector, which indicates both the direction and the magnitude of the molecular polarity. In the case of an HCl molecule, the dipole moment points from the hydrogen, the positive end, towards the chlorine, the negative end. This is usually shown with an arrow alongside the molecule, pointing towards the negative charge.
The length of this arrow can indicate the strength of the dipole moment. Dipole moments provide insights into the molecular polarity and are essential in understanding how molecules will interact in chemical reactions and solutions. They can predict which molecules will attract or repel each other, influencing the boiling and melting points of substances.
By understanding dipole moments, we gain insights into molecular behavior in various physical and chemical contexts.