Question

\(\mathrm{H}_{2} \mathrm{O}\) has net dipole moment while \(\mathrm{BeF}_{2}\) has zero dipole moment because (1) \(\mathrm{H}_{2} \mathrm{O}\) molecule is linear while \(\mathrm{Be} \mathrm{F}_{2}\) is bent (2) \(\mathrm{BeF}_{2}\) molecule is linear while \(\mathrm{H}_{2} \mathrm{O}\) is bent (3) Fluorine is more electronegative than oxygen (4) Be is more electronegative than oxygen

Short answer

Option (2): \(\text{BeF}_2\) molecule is linear while \(\text{H}_2\text{O}\) is bent.

Step-by-step solution

Understand the Concept of Dipole Moments

Dipole moments arise from differences in electronegativity between atoms in a molecule. A net dipole moment occurs when there is an uneven distribution of electron density in the molecule.

Analyze the Geometry of \(\text{H}_2\text{O}\)

\(\text{H}_2\text{O}\) has a bent or V-shaped molecular structure due to the two lone pairs of electrons on the oxygen atom, which push the hydrogen atoms closer together. This shape leads to an uneven charge distribution, resulting in a net dipole moment.

Analyze the Geometry of \(\text{BeF}_2\)

\(\text{BeF}_2\) has a linear geometry because beryllium forms two bonds with fluorine atoms and does not have lone pairs on the central atom. The symmetry of the linear shape ensures that the dipole moments of the two \(\text{Be-F}\) bonds cancel each other out, resulting in no net dipole moment.

Choose the Correct Option

Given the molecular geometries, \(\text{H}_2\text{O}\) is bent with a net dipole moment, while \(\text{BeF}_2\) is linear with no net dipole moment. Thus, the correct reason is option (2): \(\text{BeF}_2\) molecule is linear while \(\text{H}_2\text{O}\) is bent.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. Understanding the shape of a molecule is crucial as it influences physical and chemical properties, including the dipole moment.
For instance, \(\text{H}_2\text{O}\) (water) has a bent or V-shaped structure. This happens because the oxygen atom has two lone pairs of electrons. These lone pairs repel each other and push the hydrogen atoms closer together, creating a bent shape.
On the other hand, \(\text{BeF}_2\) (beryllium fluoride) has a linear structure. Beryllium, the central atom, forms two bonds with fluorine atoms and doesn't have any lone pairs. This linear geometry leads to the two \(\text{Be-F}\) bonds being opposite each other, which affects the overall dipole moment of the molecule.

Electronegativity

Electronegativity is the ability of an atom to attract shared electrons in a chemical bond. It influences how electrons are distributed in a molecule and is a key factor in determining whether a molecule will have a dipole moment.
In \(\text{H}_2\text{O}\), oxygen is more electronegative than hydrogen. This causes the shared electrons in the \(\text{O-H}\) bonds to be drawn more towards oxygen, creating partial negative and positive charges. These differences lead to an uneven distribution of electron density, contributing to the net dipole moment of water.
For \(\text{BeF}_2\), fluorine is more electronegative than beryllium. In a linear \(\text{BeF}_2\) molecule, these electronegativity differences cause dipole moments in opposite directions. However, the symmetrical linear structure ensures that the individual dipole moments cancel each other out, resulting in no net dipole moment.

Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule. It occurs when there is a difference in electronegativity between bonded atoms, leading to an uneven distribution of electron density.
The magnitude of a dipole moment depends on two factors: the difference in electronegativity between the atoms and the distance between them.
In \(\text{H}_2\text{O}\), the bent geometry causes the individual bond dipoles to add up, creating a net dipole moment. This is why water can dissolve many substances and exhibits unique chemical behaviors.
Conversely, in \(\text{BeF}_2\), the linear geometry ensures that the individual bond dipoles cancel each other out. Despite the high electronegativity difference between beryllium and fluorine, the cancelation of dipoles results in a dipole moment of zero, making \(\text{BeF}_2\) nonpolar.