The trends in melting points of the elements of \( 3^{\text{rd}} \) period are depicted in figure below
The sharp decreases observed from 'Si' to 'P' is due to
A
Decrease in atomic radius from 'Si' to 'P'
B
Change in bonding and structure of two elements
C
Different densities of two elements
D
Increase in electron density from 'Si' to 'P'
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Propolis Cognitive Error Autopsy
Official Correct Choice:
Option B: Change in bonding and structure of two elements
Concept:
The physical state and melting point of an element are dictated by its macromolecular structure and intermolecular forces.
Solution:
Silicon (Si) belongs to Group IVA and forms a three-dimensional giant covalent network structure (like diamond). Breaking this requires a massive amount of thermal energy.
Phosphorus (P) belongs to Group VA and exists as discrete, small tetra-atomic molecules (\( \text{P}_4 \)). These molecules are held together only by weak intermolecular forces (London dispersion forces).
The sharp drop in melting point is purely due to this drastic shift in bonding and structural arrangement.
Why other options are incorrect:
Radius, density, and electron density variations are minor secondary effects and do not explain the sudden massive structural collapse from a giant lattice to simple molecules.
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