Problem 159 - A Complete Resource Book in Chemistry for JEE Main

Question

When two oppositely charged ions approach each other, the ion smaller in size attracts outermost electrons of the other ion and repels its nuclear charge. The electron cloud of anion no longer remains symmetrical but is elongated towards the cation. Due to that, sharing of electrons occur between the two ions to some extent and the bond shows some covalent character. The value of dipole moment can be used for determining the amount of ionic character in a bond. Thus, percentage ionic character = \(\frac{\text { Experimental value of dipole moment }}{\text { Theoretical value of dipole moment }} \times 100\) Which one of the following compounds shows most covalent character? (a) \(\mathrm{MgCl}_{2}\) (b) \(\mathrm{AlCl}_{3}\) (c) \(\mathrm{NaCl}\) (d) All are equally covalent

Step-by-Step Solution

Answer

(b) \( \mathrm{AlCl}_{3} \) shows the most covalent character.

Step 1: Understanding Covalent and Ionic Character

The covalent character of a bond is related to the sharing of electrons between atoms. The more electrons are shared, the more covalent the bond. In contrast, ionic character involves the complete transfer of electrons. Hence, a molecule with significant covalent character will have asymmetries in the electron cloud due to polarizing effects and electron sharing.

Step 2: Using Dipole Moments to Measure Ionic Character

The given formula for percentage ionic character is \( \frac{\text{Experimental value of dipole moment}}{\text{Theoretical value of dipole moment}} \times 100 \). Molecules with higher covalent character will have a lower percentage ionic character because their experimental dipole moment will deviate more from the theoretical, due to shared electrons.

Step 3: Analyze the Ions in the Given Compounds

Consider the ions in each compound. Smaller and highly charged ions have a greater ability to polarize their neighboring ions, creating asymmetries in electron clouds, and thus increasing the covalent character. Among the given options, \( \text{Al}^{3+} \) is a smaller and highly charged cation.

Step 4: Identify the Compound with Highest Covalent Character

Among \( \text{MgCl}_2 \), \( \text{AlCl}_3 \), and \( \text{NaCl} \), the compound \( \text{AlCl}_3 \) contains \( \text{Al}^{3+} \), which significantly polarizes \( \text{Cl}^- \) ions due to its small size and high charge, leading to more sharing of electrons (covalent character), compared to \( \text{MgCl}_2 \) with \( \text{Mg}^{2+} \) and \( \text{NaCl} \) with \( \text{Na}^+ \).

Key Concepts

Covalent Character

Covalent character in a bond stems from the sharing of electrons between atoms. Unlike ionic bonds, where electrons are transferred completely from one atom to another, a covalent bond involves a mutual sharing of electron pairs. This sharing causes a partial overlap of electron clouds, which knots the atoms together more strongly.

Factors affecting covalent character include:

• Electronegativity Difference: A smaller difference in electronegativity between the atoms encourages sharing of electrons, increasing covalent character.
• Polarizability: Smaller, more charged cations have a stronger polarizing effect, pulling electron clouds towards themselves, which can create partial sharing of electrons.

Therefore, the covalent character increases when there's significant asymmetry in the electron cloud caused by these factors. When we look at compounds like \( ext{AlCl}_3\), the significant polarizing ability of \( ext{Al}^{3+}\) increases the covalent character compared to other compounds in the list.

Ionic Character

The ionic character of a compound is determined by the extent to which electrons are transferred from one atom to another in a chemical bond. The iconic ionic bond involves a complete transfer of electrons, creating ions that are held together by electrostatic forces.

However, most bonds aren't purely ionic or covalent, and many have a mix of both characteristics.

• Larger electronegative differences favor ionic character, as one atom exerts much greater pull on the electrons.
• Compounds like \( ext{NaCl}\) exhibit strong ionic character because sodium can easily transfer its outer electron to chlorine, forming \( ext{Na}^{+}\) and \( ext{Cl}^{-}\) ions.

Ionic character can be measured using dipole moments. If a bond's experimental dipole moment closely matches its theoretical value, it indicates a high ionic character. Usage of this concept reveals that \( ext{AlCl}_3\) leans more towards covalent character, unlike the other compounds.

Dipole Moment

Dipole moments are crucial in determining the polarity and ionic nature of a chemical bond. The dipole moment is a vector quantity that represents the separation of positive and negative charges in a molecule. This occurs due to the unequal sharing of electrons between atoms in a bond.

Here is how dipole moments work:

• Magnitude: The greater the separation and number of charges, the bigger the dipole moment.
• Measurement: A molecule with a high dipole moment is more polar, indicating a larger ionic character.
• Calculation: The percentage ionic character is computed by comparing experimental and theoretical dipole moments.

By comparing the experimental dipole moment against theoretical predictions, chemists can deduce the degree of ionic character in a molecule. For instance, \( ext{AlCl}_3\) despite having a substantial dipole moment, shows higher deviation from its theoretical value, indicating greater covalent character.

Polarization Effect

Polarization refers to the distortion of the electron cloud around an atom or ion, usually induced by an adjacent charged particle. The polarization effect is more pronounced with smaller and more highly charged cations.

Such cations, like \( ext{Al}^{3+}\), can pull the electron cloud of an adjacent anion like \( ext{Cl}^{-}\) towards themselves. This stretching out of the electron cloud leads to an increase in electron sharing, or covalent character, of the bond.

• Polarizing Power: It increases with the charge density on the cation.
• Impact: Greater polarization causes more overlap between electron clouds of the ions, leading to partial covalent character.
• Examples: Molecules where the cation has a small radius and high charge, like \( ext{AlCl}_3\), exhibit more significant polarization effects.

The interesting interplay between cations and electron clouds makes the study of polarization effects crucial in understanding bond character in compounds.