Which of the following pair shows abnormal low melting points?

Manganese and technetium

The correct answer is option 1. Manganese and technetium.

The pair that shows abnormally low melting points is Manganese and technetium.

Manganese (Mn) and Technetium (Tc) are both d-block elements that show abnormally low melting points compared to their neighboring elements in the periodic table. This anomaly can be attributed to their unique electronic configurations and the resulting metallic bonding.

Melting Points:

Manganese (Mn): The melting point of manganese is about 1246°C. It is unusually low compared to other transition metals like chromium (1907°C) and iron (1538°C).

Technetium (Tc): The melting point of technetium is about 2157°C, which is also lower than many of its neighboring elements in the periodic table.

Factors Contributing to Low Melting Points:

1. Electronic Configuration:

Manganese has an electronic configuration of \([Ar] 3d^5 4s^2\). The half-filled d-subshell contributes to weaker metallic bonding.

Technetium has an electronic configuration of \([Kr] 4d^5 5s^2\). Similar to manganese, the half-filled d-subshell affects the strength of metallic bonds.

2. Metallic Bonding:

The strength of metallic bonds is influenced by the number of unpaired electrons available for bonding. Manganese and technetium, with their specific configurations, result in weaker metallic bonds compared to their neighboring transition metals.

Other Pairs:

Chromium and Iron: Both elements have relatively high melting points due to strong metallic bonds.

Gold and Silver: These elements have high melting points typical of transition metals with strong metallic bonds.

Platinum and Palladium: Both have high melting points, characteristic of transition metals with strong metallic bonding.

Therefore, the pair Manganese and Technetium shows abnormally low melting points relative to their neighboring transition metals.