Which of the following transition elements of \(3d\) series has the lowest melting point?

Mn (At. No - 25)

The correct answer is option 4. Mn (At. No - 25).

The melting points of transition metals in the \(3d\) series generally depend on the strength of metallic bonding, which is influenced by several factors:

Number of Unpaired Electrons: Transition metals have unpaired electrons in their \(d\) orbitals. These unpaired electrons contribute to metallic bonding by delocalizing across the metal lattice, which strengthens the metallic bonds and increases the melting point.

Crystal Structure: The crystal structure of a metal also plays a significant role. Different structures (e.g., body-centered cubic (BCC), face-centered cubic (FCC), hexagonal close-packed (HCP)) affect how atoms are packed together, influencing the metal's melting point.

Metallic Bonding Strength: The strength of metallic bonds is related to the number of valence electrons that can participate in bonding. Metals with a higher number of valence electrons typically have stronger metallic bonds, leading to higher melting points.

Analysis of the \(3d\) Series Metals:

1. Titanium (Ti, Atomic Number 22):

Electronic Configuration: \( [Ar] 3d^2 4s^2 \)

Titanium has four valence electrons that can participate in metallic bonding, leading to a strong metallic bond and a high melting point.

2. Vanadium (V, Atomic Number 23):

Electronic Configuration: \( [Ar] 3d^3 4s^2 \)

Vanadium, with five valence electrons, forms strong metallic bonds, resulting in a high melting point.

3. Chromium (Cr, Atomic Number 24):

Electronic Configuration: \( [Ar] 3d^5 4s^1 \)

Chromium has a half-filled \(3d\) subshell, which is particularly stable and contributes to strong metallic bonding, giving chromium a high melting point.

4. Manganese (Mn, Atomic Number 25):

Electronic Configuration: \( [Ar] 3d^5 4s^2 \)

Manganese has a half-filled \(3d\) subshell, like chromium, but the bonding in manganese is weaker due to the crystal structure and the way its atoms are packed in the metal lattice. Manganese forms a complex crystal structure that is less efficient in packing compared to the other transition metals, leading to weaker metallic bonds and, consequently, a lower melting point.

Why Does Manganese Have the Lowest Melting Point?

Electronic Configuration and Bonding: Manganese's \(3d^5 4s^2\) configuration does not lead to particularly strong metallic bonding because, despite the half-filled \(d\) subshell, the electron pairing reduces the extent of bonding interaction.

Crystal Structure: Manganese crystallizes in a complex crystal structure (complex cubic), which is less stable and leads to weaker metallic bonds compared to the simpler structures (e.g., BCC or FCC) found in other transition metals.

Resulting Melting Point: The combination of a less effective metallic bonding and a complex crystal structure results in manganese having the lowest melting point among the elements listed.

Conclusion: Manganese (Mn) has the lowest melting point among Ti, V, Cr, and Mn due to its less effective metallic bonding and complex crystal structure, which reduce the stability of the metal lattice.