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

Question

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

(b) BeCl_{2} is least ionic out of the given chlorides.

Step 1: Understanding Covalent and Ionic Character

To determine the covalent and ionic character of the chlorides, we use Fajans' rules. Smaller cation size and higher charge density lead to greater polarization, increasing covalent character. Lighter metals like Be in BeCl _{2} create more covalent bonds due to higher charge density compared to heavier metals like Ba.

Step 2: Evaluate BeCl_2's Ionic Character

Among the chlorides, BeCl _{2} is expected to be the least ionic due to the small size and high charge density of the Be ^{2+} ion. This promotes polarization, increasing covalency compared to the other chlorides.

Step 3: Comparing Melting Points

Ionic compounds generally have high melting points. BeCl _{2}, having more covalent character, will have a lower melting point than the more ionic MgCl _{2}, CaCl _{2}, and BaCl _{2}. Thus, BeCl _{2} will not have the highest melting point.

Step 4: Assessing Ionic Nature

Despite some covalent character, MgCl _{2}, CaCl _{2}, and BaCl _{2} are primarily ionic due to their lower charge density and larger cation size, while BeCl _{2} is the most covalent.

Key Concepts

Covalent Character

Covalent character in compounds refers to the degree to which bonds resemble covalent rather than ionic bonds. According to Fajans' rules, the covalent nature of a bond increases with the ability of a cation to polarize an anion. This polarization leads to electron cloud distortion, making the bond more covalent. For metal chlorides like \(\text{BeCl}_2\), \(\text{MgCl}_2\), \(\text{CaCl}_2\), and \(\text{BaCl}_2\), the covalent character depends largely on the following factors:

• Cation size: Smaller cations have higher charge density, leading to greater polarization of the anion.
• Charge: A higher positive charge on the cation increases its polarization power.
• Polarization effects: Metals such as beryllium (Be) can cause significant polarization due to their small size and high charge density, giving compounds like \(\text{BeCl}_2\) a stronger covalent character compared to those with larger and less charged cations like barium.

In general, lighter metals tend to form compounds with higher covalent character due to their ability to polarize negatively charged ions more effectively.

Ionic Compounds

Ionic compounds result from the complete transfer of electrons from a metal to a non-metal, forming ions that are held together by electrostatic forces. These compounds typically exhibit high melting and boiling points owing to the strong attraction between oppositely charged ions. In the case of metal chlorides, such as \(\text{MgCl}_2\), \(\text{CaCl}_2\), and \(\text{BaCl}_2\), they exhibit significant ionic character because:

• The metal atoms lose electrons easily to form positive ions.
• The chloride ions provide a significant electrostatic attraction due to their negative charge.
• Larger metal ions with lower polarization power also contribute to strong ionic bonds, which results in higher melting points.

However, despite being ionic, these compounds may showcase some degree of covalent character. This is especially the case if the cation can polarize the anion to any considerable extent, as is somewhat seen in \(\text{BeCl}_2\). Nonetheless, \(\text{MgCl}_2\), \(\text{CaCl}_2\), and \(\text{BaCl}_2\) are considered predominantly ionic.

Metal Chlorides

Metal chlorides are compounds formed between metallic elements and chloride ions. These compounds vary in their nature from ionic to covalent depending on several factors:

• The metal's position in the periodic table affects chloride formation. Lighter metals often form chlorides with more covalent character.
• Smaller and highly charged metal ions such as \(\text{Be}^{2+}\) in \(\text{BeCl}_2\) lead to greater covalency.
• Heavier metal ions like \(\text{Ba}^{2+}\), as seen in \(\text{BaCl}_2\), form chlorides that display more ionic characteristics.

Considering these factors, \(\text{BeCl}_2\) is a notable example of a metal chloride with relatively high covalent character due to beryllium's small size and high charge. This affects its physical properties, such as melting point, making it lower than more ionic equivalents. Thus, understanding the balance of ionic and covalent character in metal chlorides is crucial for predicting their chemical and physical properties.