Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

The halide having the highest melting point is (a) \(\mathrm{NaF}\) (b) \(\mathrm{NaCl}\) (c) \(\mathrm{NaBr}\) (d) NaI

Step-by-Step Solution

Answer

96NaF
97 has the highest melting point due to its small ionic size and strong ionic bonds.

Step 1: Identify the Concept

In this exercise, we need to determine which sodium halide (6NaF
97, 6NaCl
97, 6NaBr
97, 7NaI
97) has the highest melting point by understanding how melting points are affected by the nature of the ionic bonds involved.

Step 2: Understand Ionic Bond Strength

The strength of an ionic bond depends on the charges of the ions and the distance between their nuclei. Smaller ions will form stronger ionic bonds due to closer proximity of opposite charges, resulting in higher melting points.

Step 3: Compare Ionic Sizes

The ionic size increases from 6NaF
97 to 6NaI
97, because the halogen ions (F6-
99, Cl6-
99, Br6-
99, I6-
99) increase in size down the group in the periodic table. Therefore, fluoride ions are the smallest, followed by chloride, bromide, and finally iodide ions.

Step 4: Determine Melting Points Based on Ionic Size

With 6NaF
97 having the smallest anion (F6-
99), it forms the strongest ionic bonds among the options listed due to minimal distance between the nuclei, leading to the highest melting point.

Key Concepts

Ionic Bond Strength

Ionic bonds are formed between positively and negatively charged ions. Their strength is primarily influenced by two factors: the magnitude of the charges and the distance between them. Generally, a higher charge leads to a stronger bond. However, when ions have the same charge (as in the case of halides), the distance between the ions becomes key.

• Smaller ions have their centers of charge closer together.
• This closeness enhances the attractive forces between the ions.
• Stronger attractive forces result in a stronger ionic bond.

This explains why ionic compounds made from smaller ions, like fluoride ions in sodium fluoride (\(\mathrm{NaF}\)), have higher melting points. The tighter the bond, the more energy is required to break it during melting.

Ionic Sizes in Halides

Halides are a group of ions derived from halogen elements. These include fluoride (\(\mathrm{F^-}\)), chloride (\(\mathrm{Cl^-}\)), bromide (\(\mathrm{Br^-}\)), and iodide (\(\mathrm{I^-}\)). As we move down the group in the periodic table, the size of these ions increases.
This increase in size is due to the addition of electron shells, which places the outermost electrons farther from the nucleus. As a result:

• Fluoride ions are the smallest, leading to the strongest ionic bonds with sodium ions.
• Chloride ions are larger, followed by bromide ions.
• Iodide ions are the largest, causing weaker ionic interactions due to increased distance between oppositely charged nuclei.

Thus, the smaller the anion in the halide, the stronger the resulting ionic bond. This directly impacts the compound's melting point.

Periodic Trends in Halides

Understanding periodic trends is crucial for predicting properties like melting points of ionic compounds. In the case of halides, these trends help explain why the melting points vary as you move down the halogen group in the periodic table.
The periodic trend observed in halides is primarily due to changes in ionic size, as mentioned earlier. As you descend the group:

• The size of the halide ions increases with each step down the group.
• Larger ions result in weaker ionic bonds due to increased distance between ions.
• This makes it easier to separate the ions, thereby lowering the melting point.

For sodium halides, \(\mathrm{NaF}\), with the smallest halogen ion, exhibits the highest melting point. On the contrary, \(\mathrm{NaI}\), with the largest, has the lowest. This clear trend highlights the predictable nature of periodic behavior in chemistry.