Question

When anions and cations approach each other, the valence shell of anions are pulled towards cation nucleus and thus, shape of anion is deformed. The phenomenon of deformation of anion by a cation is known as polarization and the ability of the cation to polarize the anion is called as polarizing power of cation. Due to polarization, sharing of electrons occurs between two ions to some extent and the bond shows some covalent character. Which is most volatile in nature? (a) \(\mathrm{AlCl}_{3}\) (b) \(\mathrm{CaCl}_{2}\) (c) \(\mathrm{NaCl}\) (d) \(\mathrm{MgCl}_{2}\)

Short answer

AlCl3 is the most volatile due to its high covalent character.

Step-by-step solution

Understand Polarization

When a cation exerts an attractive force causing distortion in the electron cloud of the anion, it leads to polarization. The extent of polarization affects the bond nature, increasing its covalent character.

Identify Polarizing Power

The polarizing power of a cation depends on its charge and size. Smaller and higher charge cations have stronger polarizing power. Aluminum ion (8 ext{Al}^{3+}9) with a small size and +3 charge has high polarizing power compared to others like 8 ext{Ca}^{2+}9, 8 ext{Na}^{+}9, and 8 ext{Mg}^{2+}9.

Determine Covalent Character

More polarizing power increases covalent character due to increased overlap of electron clouds. Therefore, a compound with cations having higher polarizing power will exhibit more covalent character.

Evaluate Volatility

Compounds with more covalent character generally exhibit higher volatility. 8 ext{AlCl}_{3}9, having highly polarizing and consequently more covalent character, is expected to be more volatile than ionic compounds 8 ext{CaCl}_{2}9, 8 ext{NaCl}9, and 8 ext{MgCl}_{2}9.

Conclusion

Based on the analysis, 8 ext{AlCl}_{3}9 is predicted to be the most volatile compound due to its higher covalent character influenced by its cation's polarizing power.

Key concepts

Polarizing Power of Cations

When discussing polarization in chemistry, it is essential to understand the concept of the polarizing power of cations. Cations, which are positively charged ions, can cause distortion in the electron cloud of nearby anions, which are negatively charged ions. This distortion is called polarization. The degree to which a cation can distort an anion's electron cloud depends largely on two factors:

• Size of the Cation: Smaller cations can get closer to the anion and exert a stronger attractive force. This increases their ability to polarize.
• Charge of the Cation: Cations with a higher positive charge have a stronger electric field around them, enhancing their polarizing power. For example, \( \text{Al}^{3+}\) ions have a higher polarizing power compared to \( \text{Na}^{+}\) because of the greater charge.

The polarizing power of a cation plays a crucial role in determining the characteristics of the bonds it forms. Understanding this can provide deep insights into the nature of the compound's structure and properties.

Covalent Character

As cations with high polarizing power approach anions, they cause a degree of electron sharing in the normally ionic bond. This sharing of electrons is what gives the bond its covalent character. Covalent character refers to the extent to which electrons are shared between atoms in a bond, similar to covalent bonds where electrons are shared more equally.In compounds where the cation has a high polarizing power, such as \( \text{AlCl}_{3}\), the electron cloud of the anion is distorted to a significant extent, leading to increased overlap of electron clouds. This overlap blurs the distinction between a purely ionic and purely covalent bond, resulting in covalent character.

• Higher covalent character implies that the compound behaves more like a covalent compound.
• Compounds with greater covalent character often have different physical properties compared to purely ionic compounds, such as lower melting points and different solubility in water.

Covalent character is thus a vital concept in predicting the behavior and properties of a chemical compound.

Volatility of Compounds

Volatility refers to how easily a substance can vaporize. It is an important property in chemistry and relates to how strong or weak the bonds in a compound are.Compounds that display more covalent character are generally more volatile than those that are predominantly ionic. This is because covalent bonds, while strong when considering purely covalent compounds, are weaker in compounds with a covalent character when compared to the traditionally stronger ionic bonds.

• Compounds like \( \text{AlCl}_{3}\) display high volatility due to their significant covalent character, influenced by the high polarizing power of the \( \text{Al}^{3+}\) ion. Their molecule-like nature means they readily evaporate.
• Ionic compounds such as \( \text{NaCl}\), \( \text{CaCl}_{2}\), and \( \text{MgCl}_{2}\) typically have strong electrostatic forces holding them together, leading to lower volatility.

Understanding the relationship between covalent character and volatility helps in predicting the behavior of substances under different conditions. This knowledge is especially valuable in industrial applications where control over evaporation and other physical properties is critical.