Question
Question: Which of the following order is correct for melting point of d-block elements?...
Which of the following order is correct for melting point of d-block elements?
A
Cr > Mo > W
B
Mo > Nb > Zr
C
Mn > Fe > Co
D
Os > Re > W
Answer
Mo > Nb > Zr
Explanation
Solution
Melting points of d-block elements are determined by the strength of metallic bonding, which depends on the number of unpaired electrons and the extent of d-orbital overlap. Generally, melting points increase across a period up to the middle (e.g., Cr, Mo, W) and tend to increase down a group.
- Option A (Cr > Mo > W): Elements of Group 6. Melting points increase down the group: W (3422°C) > Mo (2623°C) > Cr (1907°C). This order is incorrect.
- Option B (Mo > Nb > Zr): Elements of Period 5, Groups 6, 5, and 4 respectively. Melting points increase across the period: Mo (2623°C) > Nb (2477°C) > Zr (1852°C). This is due to an increasing number of valence electrons (4d and 5s) and unpaired electrons contributing to stronger metallic bonding. This order is correct.
- Option C (Mn > Fe > Co): Elements of the 3d series. The melting point trend is Fe (1538°C) > Co (1495°C) > Mn (1246°C). This order is incorrect.
- Option D (Os > Re > W): Elements of the 6th period, Groups 8, 7, and 6. Melting points decrease across the period from Group 6 to Group 8: W (3422°C) > Re (3186°C) > Os (3033°C). This order is incorrect.
Therefore, the correct order is Mo > Nb > Zr.