Problem 96 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{H}_{2} \mathrm{O}\) is dipolar, whereas \(\mathrm{BeF}_{2}\) is not. It is because (a) the electronegativity of \(\mathrm{F}\) is greater than that of \(\mathrm{O}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) involves hydrogen bonding where as \(\mathrm{BeF}_{2}\) is a discrete molecule (c) \(\mathrm{H}_{2} \mathrm{O}\) is linear and \(\mathrm{BeF}_{2}\) is angular (d) \(\mathrm{H}_{2} \mathrm{O}\) is angular and \(\mathrm{BeF}_{2}\) is linear

Step-by-Step Solution

Answer

Option (d): \(\mathrm{H}_{2} \mathrm{O}\) is angular, while \(\mathrm{BeF}_{2}\) is linear.

Step 1: Identify Molecular Structures

To understand why one molecule is dipolar and the other is not, first identify their molecular structures. \(\mathrm{H}_{2} \mathrm{O}\) has a bent (angular) shape due to the two lone pairs of electrons on the oxygen. \(\mathrm{BeF}_{2}\), on the other hand, is a linear molecule because there are no lone pairs on the beryllium atom.

Step 2: Evaluate Dipole Moment

A molecule is dipolar (has a dipole moment) when there is an uneven distribution of electron charge due to differences in electronegativity and the shape of the molecule. In \(\mathrm{H}_{2} \mathrm{O}\), the bent shape causes the dipole moments from the O-H bonds to not cancel out, creating a net dipole moment. In \(\mathrm{BeF}_{2}\), the molecule is linear, so the dipole moments from the Be-F bonds cancel each other out, resulting in no net dipole moment.

Step 3: Contrast Options with Identified Characteristics

Compare the structures with the given options. - (c) is incorrect as it describes \(\mathrm{H}_{2} \mathrm{O}\) as linear and \(\mathrm{BeF}_{2}\) as angular, which is the opposite of their true shapes. - (d) correctly describes \(\mathrm{H}_{2} \mathrm{O}\) as angular and \(\mathrm{BeF}_{2}\) as linear, aligning with the identified structures and explaining the dipole characteristics.

Key Concepts

Dipole Moment

A dipole moment occurs when there is an uneven distribution of electron charge in a molecule. This happens due to differences in electronegativity between atoms and the shape of the molecule. Think of it as having a partial positive charge on one side and a partial negative charge on the other side. In simple terms, it's like a small magnet inside the molecule.

For example:

• Molecules like water (\(\mathrm{H}_{2}\mathrm{O}\)) have a net dipole moment, because the bent shape doesn't allow the charge differences to cancel out.
• In contrast, molecules such as beryllium fluoride (\(\mathrm{BeF}_{2}\)) are linear, so their charge differences can cancel out, resulting in no net dipole moment.

Understanding whether a molecule has a dipole moment is essential in predicting its interactions with other molecules, especially its solubility and reactivity.

Molecular Geometry

Molecular geometry is all about the shape of a molecule. It is determined by the arrangement of atoms and the lone pairs of electrons around the central atom. This shape affects the molecule's physical and chemical properties.

Let's break it down:

• Water (\(\mathrm{H}_{2}\mathrm{O}\)) has a bent or angular shape. This occurs because the two lone pairs of electrons on the oxygen atom push the hydrogen atoms away, creating a bent structure.
• Beryllium fluoride (\(\mathrm{BeF}_{2}\)) has a linear shape with the two fluorine atoms directly opposite each other. This is because beryllium doesn't have lone pairs of electrons to create a bend.

Understanding molecular geometry helps us predict how molecules behave, including how they interact with each other and their environment.

Electronegativity

Electronegativity is a measure of how strongly an atom can attract electrons in a chemical bond. It varies across the periodic table and plays a crucial role in determining the polarity of molecules.

Consider these points:

• Oxygen is more electronegative than hydrogen, which means it pulls the bonding electrons closer to itself in the \(\mathrm{H}_{2}\mathrm{O}\) molecule. This creates a partial negative charge around the oxygen and a partial positive charge around the hydrogen atoms.
• Fluorine is more electronegative than beryllium, but because \(\mathrm{BeF}_{2}\) is linear, the forces created by this electronegativity cancel out.

Knowing the electronegativity of elements in a compound helps in predicting the bond type and the molecule's overall polarity. It's an essential factor in determining whether a molecule is polar or nonpolar.

Chemical Bonding

Chemical bonding is the process that holds atoms together in molecules. There are different types of bonds, and understanding these can help explain the behavior and properties of substances.

Important points to remember:

• Covalent bonds involve the sharing of electron pairs between atoms. For example, both \(\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{BeF}_{2}\) have covalent bonds, although their properties differ because of geometry and electronegativity differences.
• The nature of the chemical bond influences the molecule's polarity. In water, the polar covalent bonds create a situation where the electron distribution is uneven, resulting in a polar molecule.

Chemical bonding not only determines the stability of molecules but also explains how they interact with other substances, which is fundamental to chemical reactions.