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

(d) \(\mathrm{H}_{2} \mathrm{O}\) is angular and \(\mathrm{BeF}_{2}\) is linear.

Step 1: Identify Molecular Shapes

First, determine the shapes of the molecules for both \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{BeF}_{2}\). \(\mathrm{H}_{2} \mathrm{O}\) has a bent (angular) molecular shape, while \(\mathrm{BeF}_{2}\) is linear.

Step 2: Analyze Polarity in Molecules

A molecule is polar if it has a net dipole moment. In \(\mathrm{H}_{2} \mathrm{O}\), the bent shape causes an uneven distribution of charge, resulting in a net dipole moment. For \(\mathrm{BeF}_{2}\), the linear shape results in the dipole moments of the \(\mathrm{Be-F}\) bonds cancelling each other out, making it nonpolar.

Step 3: Examine Option Logic

Given the polar nature of \(\mathrm{H}_{2} \mathrm{O}\) due to its shape and the nonpolar nature of \(\mathrm{BeF}_{2}\) due to its linear shape, option (d) accurately describes this difference: "\(\mathrm{H}_{2} \mathrm{O}\) is angular and \(\mathrm{BeF}_{2}\) is linear."

Step 4: Conclusion: Choose the Correct Answer

Based on the analysis, the reason for \(\mathrm{H}_{2} \mathrm{O}\) being dipolar and \(\mathrm{BeF}_{2}\) not being dipolar is because \(\mathrm{H}_{2} \mathrm{O}\) is angular and \(\mathrm{BeF}_{2}\) is linear.

Key Concepts

Dipole Moment

A dipole moment occurs when there is a separation of charges within a molecule. This separation happens because one end of the molecule has a slight excess of negative charge, while the other end has a slight excess of positive charge.
The magnitude of the dipole moment depends on the difference in electronegativity between the atoms and the distance between them. In a molecule like \(\text{H}_2\text{O}\), the bent structure causes an uneven distribution of charge, creating a strong dipole moment.

• This dipolar nature arises because the oxygen atom is more electronegative than the hydrogen atoms, pulling electrons closer to itself.
• In contrast, \(\text{BeF}_2\) has a linear shape.
• This shape allows the dipole moments from each \(\text{Be-F}\) bond to cancel each other out, resulting in no net dipole moment.

Thus, while one molecule might be polar, another with a different shape can be nonpolar despite having similar atomic components.

Molecular Polarity

Molecular polarity is determined by both the shape of the molecule and the difference in electronegativity between its atoms.
If a molecule has a net dipole moment, it is considered polar.
For a molecule to be polar, it needs to have polar bonds that do not cancel each other out. This often leads to asymmetric molecules.
In \(\text{H}_2\text{O}\), the bent shape and the electronegativity difference between hydrogen and oxygen generate a net dipole moment, making the molecule polar.

• The angles between hydrogen atoms in water point in different directions.
• This results in a distribution of charges that is not balanced.

On the other hand, \(\text{BeF}_2\), being linear, has bond dipoles that cancel each other. Hence, it is nonpolar even though the bonds themselves are polar due to the electronegativity difference between beryllium and fluorine.

Electronegativity

Electronegativity is the ability of an atom to attract electrons towards itself.
It plays a crucial role in determining the polarity of molecules.
In the context of \(\text{H}_2\text{O}\) and \(\text{BeF}_2\), we see different outcomes due to variances in electronegativity.

• Oxygen has a higher electronegativity compared to hydrogen, causing a partial negative charge around the oxygen atom in water.
• With beryllium and fluorine, despite fluorine having high electronegativity, the linear structure of \(\text{BeF}_2\) makes the pull of electrons from each fluorine cancel out.

This property not only affects whether a molecule is polar or nonpolar, but can also influence its reactivity and bonding interactions with other molecules.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction involving hydrogen atoms that are bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.
This leads to stronger intermolecular attractions compared to regular dipole interactions.

• In \(\text{H}_2\text{O}\), hydrogen bonding is significant due to the presence of hydrogen atoms bonded to the highly electronegative oxygen atom.
• This interaction leads to water's high boiling and melting points compared to other molecules of similar size.

Despite the presence of polar \(\text{Be-F}\) bonds in beryllium fluoride, \(\text{BeF}_2\) does not participate in hydrogen bonding. The lack of hydrogen atoms bound to electronegative atoms in \(\text{BeF}_2\) precludes the formation of such interactions.