Question

Arrange the following in increasing order of covalent character \(-\mathrm{NaCl}, \mathrm{MgCl}_{2}, \mathrm{AlCl}_{3}\). (a) \(\mathrm{NaCl}<\mathrm{MgCl}_{2}<\mathrm{AlCl}_{3}\) (b) \(\mathrm{MgCl}_{2}<\mathrm{NaCl}<\mathrm{ACl}_{3}\) (c) \(\mathrm{AlCl}_{3}<\mathrm{MgCl}_{2}<\mathrm{NaCl}\) (d) \(\mathrm{NaCl}<\mathrm{AICl}_{3}<\mathrm{MgCl}_{2}\)

Short answer

The compounds arranged in increasing order of covalent character are (a) \text{NaCl}<\text{MgCl}_{2}<\text{AlCl}_{3}

Step-by-step solution

Understand Covalent Character

The covalent character of an ionic compound is related to the polarizing power of the cation. A cation with a higher charge and a smaller size polarizes the electron cloud of the anion more, leading to a greater covalent character. Fajans' rules help predict this.

Comparing Charges of the Cations

First, compare the charges of the cations in the given compounds. Na+ has a +1 charge, Mg2+ has a +2 charge, and Al3+ has a +3 charge. A higher charge tends to increase the polarizing power of the cation.

Comparing Sizes of the Cations

Next, compare the sizes of the cations. In a group on the periodic table, as the atomic number increases, the size of the cations increases due to an increase in the number of electron shells. Na+ is larger than Mg2+ which is in turn larger than Al3+.

Determine the Covalent Character Order

Using Fajans' rules and considering both charge and size, we can predict that AlCl3 will have the highest covalent character due to the small size and +3 charge of Al3+, followed by MgCl2 with a +2 charge and then NaCl with a +1 charge and largest size.

Key concepts

Fajans' Rules

Understanding Fajans' Rules is vital to grasp why certain ionic compounds exhibit varying degrees of covalent character. These rules are a set of guidelines that predict the likelihood of an ionic compound having covalent properties.

At their core, Fajans' Rules consider the charge and size of ions. A cation with a higher positive charge will exert a stronger electrostatic pull on the electrons of a nearby anion. This can cause a distortion in the electron cloud of the anion, a phenomenon known as polarization.

Similarly, the size of the cation plays a significant role. Smaller cations have higher charge densities and can polarize anions more effectively than larger ones. This is because the positive charge is concentrated over a smaller area, leading to a stronger attraction for the electrons of the anion.

In conclusion, according to Fajans' Rules, a compound will have a significant covalent character if it has a cation with a high charge and a small radius, as these factors contribute to stronger polarizing power.

Ionic Compounds

Ionic compounds are chemical compounds composed of ions—charged particles—that are held together by ionic bonds. These bonds are the result of the attraction between positively charged cations and negatively charged anions.

In a typical ionic compound, a metal atom donates one or more electrons to a nonmetal atom, resulting in the metal atom becoming a cation and the nonmetal a negatively charged anion. The most straightforward example of such a compound is sodium chloride (NaCl), where sodium (Na) donates an electron to chlorine (Cl), creating Na+ and Cl- ions.

While ionic compounds are primarily characterized by ionic bonding, it is essential to understand that the nature of chemical bonding is not always ‘black and white.’ In reality, many ionic compounds can display some degree of covalent character—where electrons are not fully transferred but are shared to some extent. This is largely influenced by the polarizing power of the cations involved, as described by Fajans' Rules.

Polarizing Power

Polarizing power is a concept that underscores an ion's ability to distort the electron cloud of another ion, which typically leads to increased covalent character in the bond. It is a crucial piece of the puzzle in figuring out why certain compounds have more ionic or covalent bonding.

For instance, a cation's polarizing power is heavily influenced by its charge and size. A high charge and small size mean a cation can wield significant polarizing power, pulling the electron cloud of its neighboring anion towards itself. This results in electron-sharing characteristics, typical of covalent bonding, even within a compound typically classified as ionic.

The principle of polarizing power is effectively illustrated when comparing ionic compounds like NaCl, MgCl₂, and AlCl₃. In this scenario, Al³⁺ has a significantly higher polarizing power than Na⁺ or Mg²⁺, therefore AlCl₃ is expected to exhibit the greatest covalent character according to Fajans' Rules, followed by MgCl₂ and lastly NaCl.