Chemistry S & P Block Elements MDCAT 2013
PMDC Verified Question 96 of 104
What is the trend of melting and boiling points of the elements of short periods as we move from left to right in a periodic table?
A
Melting and boiling points decrease gradually
B
Melting and boiling points first decrease then increase
C
Melting and boiling points increase gradually
D
Melting and boiling points first increase then decrease
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: Melting and boiling points first increase then decrease
Concept:

Melting and boiling points depend heavily on the strength of binding forces, which in turn depend on the number of valence electrons available for bonding and the type of structure formed.

Solution:

  • Moving left to right across a short period, the number of valence electrons increases from Group IA to IVA. This increases metallic/covalent binding strength, so melting/boiling points increase.
  • Carbon/Silicon in Group IVA form giant covalent structures, reaching the maximum melting point.
  • From Group VA onwards, elements form small discrete molecules (like \( \text{N}_2 \), \( \text{O}_2 \), \( \text{Cl}_2 \)) held together only by weak London dispersion forces, causing the melting/boiling points to sharply decrease.


Why other options are incorrect:

  • The trend is not gradual or strictly one-directional; it is a curve that peaks at Group IVA.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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