JEE Chemistry 2025 Question with Solution

Given: We need the correct order of melting points for the pairs $\text{(Mn, Fe), (Tc, Ru)}$ and $\text{(Re, Os)}$.

Find: Which option matches the correct increasing order.

The solution explains that for transition metals, melting point mainly depends on the strength of metallic bonding. Stronger metallic bonding generally arises from greater participation of $\text{(n-1)d}$ and $\text{ns}$ electrons.

For the first pair:

$$\text{Mn} < \text{Fe}$$

Mn has relatively weaker metallic bonding than Fe.

For the second pair:

$$\text{Tc} < \text{Ru}$$

Ru has stronger metallic bonding than Tc.

For the third pair:

$$\text{Re} < \text{Os}$$

Os has stronger metallic bonding than Re.

Therefore, the correct overall order is:

$$\text{Mn} < \text{Fe}, \quad \text{Tc} < \text{Ru}, \quad \text{and} \quad \text{Re} < \text{Os}$$

This matches option C. The correct option is C.

Given: Compare the melting points of $\text{Mn}$ and $\text{Fe}$, $\text{Tc}$ and $\text{Ru}$, and $\text{Re}$ and $\text{Os}$.

Find: The option that gives the correct order.

Principle: The melting point of transition metals depends on metallic bond strength, which is influenced by the participation of d-electrons and the number of unpaired electrons available for bonding.

1. For $\text{Mn}$ and $\text{Fe}$:

$$\text{Mn} : [\text{Ar}]\, 3d^5 4s^2$$ $$\text{Fe} : [\text{Ar}]\, 3d^6 4s^2$$

Mn has a relatively lower melting point, while Fe shows stronger metallic bonding. Hence,

$$\text{Mn} < \text{Fe}$$

2. For $\text{Tc}$ and $\text{Ru}$: Ru has stronger metallic bonding than Tc. Hence,

$$\text{Tc} < \text{Ru}$$

3. For $\text{Re}$ and $\text{Os}$: Os has stronger metallic bonding than Re. Hence,

$$\text{Re} < \text{Os}$$

Combining all three comparisons:

$$\text{Mn} < \text{Fe}, \quad \text{Tc} < \text{Ru}, \quad \text{and} \quad \text{Re} < \text{Os}$$

Therefore, the correct option is C.