Question

Which of the following molecules has a zero dipole moment? (a) \(\mathrm{BeCl}_{2}\) (b) \(\mathrm{HCl}\) (c) \(\mathrm{NH}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

(a) \(\mathrm{BeCl}_{2}\) has a zero dipole moment.

Step-by-step solution

Understanding Dipole Moment

The dipole moment of a molecule is a measure of the separation of positive and negative charges within the molecule. A molecule with a zero dipole moment is nonpolar, meaning its charge distribution is symmetrical, and the dipole vectors cancel each other out.

Analysis of Each Molecule's Geometry

Analyze the molecular geometry of each option to determine if the dipole moments cancel out:- (a) \(\mathrm{BeCl}_{2}\) is linear, with the central \(\mathrm{Be}\) atom bonded symmetrically to two \(\mathrm{Cl}\) atoms, making it nonpolar.- (b) \(\mathrm{HCl}\) is linear but asymmetrical, resulting in a dipole moment because of the difference in electronegativity between H and Cl.- (c) \(\mathrm{NH}_{3}\) has a trigonal pyramidal shape due to a lone pair of electrons, leading to a net dipole moment.- (d) \(\mathrm{H}_{2} \mathrm{O}\) is bent due to its two lone pairs, causing a net dipole moment.

Conclusion

Based on the molecular geometry analysis, \(\mathrm{BeCl}_{2}\) is the only molecule with a symmetrical charge distribution, resulting in a zero dipole moment.

Key concepts

Molecular Geometry

Molecular geometry is a crucial aspect of chemistry that explains the three-dimensional arrangement of atoms within a molecule. This arrangement determines many physical and chemical properties.
One common geometry is linear, where atoms are arranged in a straight line. An example is \( \text{BeCl}_2 \), which features beryllium at its center with two chlorine atoms symmetrically positioned at both ends.

• Linear molecules can have a dipole moment of zero if they're symmetrical.
• Other geometries like trigonal planar and tetrahedral have atoms positioned at specific angles.

In contrast, molecules like \( \text{NH}_3 \) form a trigonal pyramidal shape due to the influence of lone electron pairs.
Understanding molecular geometry helps predict molecule behavior, reactivities, and interactions.

Polar and Nonpolar Molecules

Molecules can be classified as either polar or nonpolar based on the distribution of their electrical charges. This property is key in determining molecule interactions.

• Polar molecules have an uneven charge distribution, leading to a non-zero dipole moment.
• Nonpolar molecules have a symmetrical charge distribution, resulting in a zero dipole moment.

An example of a polar molecule is \( \text{HCl} \), where the chlorine atom pulls electrons towards itself, creating a permanent dipole. On the other hand, \( \text{BeCl}_2 \) is an example of a nonpolar molecule. Its symmetry allows the dipole forces from the chlorine atoms to cancel out perfectly.
Recognizing whether a molecule is polar or nonpolar is essential for understanding its solubility, boiling point, and how it will interact with other substances.

Symmetrical and Asymmetrical Charge Distribution

The charge distribution within a molecule determines its overall polarity. Here are the key points:

• Symmetrical charge distribution occurs when the molecule's shape allows for equal charge dispersion.
• Asymmetrical charge distribution happens when there is an uneven spread of charges.

A molecule with a symmetrical charge distribution, like \( \text{BeCl}_2 \), is nonpolar, because its symmetrical shape facilitates no overall separation of charges.
On the contrary, asymmetrical molecules such as \( \text{H}_2\text{O} \) have regions with different charges due to their bent shape and lone pairs of electrons, resulting in a polar molecule with a net dipole moment. Understanding the type of charge distribution is fundamental in predicting the behavior and properties of molecules in different chemical contexts.