Chemistry S & P Block Elements PMDC 2020
PMDC Verified Question 67 of 104
Magnesium reacts with nitrogen to form
A
\( \text{Mg}_2\text{N}_2 \)
B
\( \text{Mg}_3\text{N}_2 \)
C
\( \text{MgN}_2 \)
D
MgN
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: \( \text{Mg}_3\text{N}_2 \)
Concept:

When writing the chemical formula for an ionic compound, the total positive charge must perfectly balance the total negative charge (the criss-cross method).

Solution:

  • Magnesium belongs to Group II-A and always forms a cation with a +2 charge: \( \text{Mg}^{2+} \).
  • Nitrogen belongs to Group V-A, meaning it needs 3 electrons to complete its octet, forming the nitride anion with a -3 charge: \( \text{N}^{3-} \).
  • Applying the criss-cross method to balance the charges:
    1. Take the magnitude of Mg's charge (2) and make it the subscript for N.
    2. Take the magnitude of N's charge (3) and make it the subscript for Mg.
  • The resulting balanced formula is \( \text{Mg}_3\text{N}_2 \).


Why other options are incorrect:

  • All other options represent incorrect charge balancing. For instance, \( \text{MgN}_2 \) would imply Nitrogen has a -1 charge, which is incorrect for a nitride.

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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