Problem 156 - A Complete Resource Book in Chemistry for JEE Main

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. Considering \(\mathrm{BeCl}_{2}, \mathrm{MgCl}_{2}, \mathrm{CaCl}_{2}\) and \(\mathrm{BaCl}_{2}\), predict which of the following statement is true? (a) Covalent character increases as the atomic number of the metal atom increases (b) \(\mathrm{BeCl}_{2}\) is least ionic out of the given chlorides. (c) \(\mathrm{BeCl}_{2}\) has the highest melting point among the given chlorides. (d) All are highly ionic compound.

Step-by-Step Solution

Answer

Statement (b) is true; \(\mathrm{BeCl}_{2}\) is least ionic.

Step 1: Understanding Polarization

Polarization occurs when a cation distorts the electron cloud of an anion. The ability of a cation to distort the electron cloud of an anion is called polarizing power, which depends on the cation's charge and size. Smaller and highly charged cations have higher polarizing power, leading to increased covalent character.

Step 2: Comparing Cations

Be and the other elements mentioned are metals from the alkaline earth group. As we move down the group (from Be to Ba), atomic size increases and the charge remains +2. A larger cation has weaker polarizing power compared to a smaller one, making Be, with the smallest size, the most polarizing.

Step 3: Polarizing Power and Covalent Character

Be with its small size and high polarizing power will cause more distortion in the anion's electron cloud, increasing covalent character. Therefore, compounds with Be, like \(\mathrm{BeCl}_{2}\), are less ionic and more covalent compared to those with larger cations like Ba, Ca, or Mg.

Step 4: Evaluating Statements

a) Incorrect: Covalent character increases as the polarizing power of the cation increases, which is highest for Be.b) Correct: \(\mathrm{BeCl}_{2}\) is the least ionic and most covalent.c) Incorrect: The covalent character does not necessarily correlate with higher melting points.d) Incorrect: \(\mathrm{BeCl}_{2}\) is less ionic compared to the others.

Key Concepts

Polarizing Power of Cations

When a cation and an anion are near each other, the cation can pull on the electrons of the anion, distorting its electron cloud. This effect is critical in chemistry and is known as the polarizing power of the cation. The polarizing power of a cation depends mainly on two factors:

• The charge on the cation: A higher charge increases polarizing power because the positive charge exerts a stronger pull on the anion's electrons.
• The size of the cation: Smaller cations have higher polarizing power. They can get closer to the anion and exert a stronger pull.

When a cation has high polarizing power, it can distort the anion's electron cloud significantly, leading to one key outcome: the bond between the two ions gains some covalent character, resulting in a sharing of electrons rather than just an ionic interaction.

Covalent Character

In ionic compounds, the term covalent character refers to the presence of electron sharing between a cation and an anion. While pure ionic bonds are based on complete electron transfer, polarizing power can induce a degree of electron sharing.Several factors influence the covalent character in ionic compounds:

• High polarizing power of cations: As cations distort electron clouds more, they induce covalent character.
• The size of the anion: Larger anions are more easily distorted, also increasing covalent character.

For example, in \(\mathrm{BeCl}_2\), Beryllium's small size and high charge density lead to a significant covalent character. It cannot be considered purely ionic due to the increased electron sharing.

Alkaline Earth Metals

Alkaline earth metals include elements like Be, Mg, Ca, and Ba. These metals belong to the second group of the periodic table and are highly reactive, commonly forming ionic compounds. As you move down the group from Beryllium to Barium:

• The atomic size increases.
• The cations become larger with the same +2 charge.
• The polarizing power decreases because larger cations are less effective at distorting neighboring anions.

In the context of salts like \(\mathrm{BeCl}_2\) or \(\mathrm{BaCl}_2\), Beryllium chloride is more covalent due to its smaller cation size and greater polarizing ability. This trend helps chemists predict the nature of ionic compounds formed by alkaline earth metals, such as ionic strength and melting points.