Chemistry Chemical Bonding MDCAT 2016
PMDC Verified Question 92 of 102
Choose the right molecule for the given dot and cross diagram (a central atom sharing three electron pairs and possessing one lone pair):

XHHHxxx
A
\( \text{CH}_3 \)
B
\( \text{H}_2\text{O} \)
C
\( \text{CO} \)
D
\( \text{NH}_3 \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: \( \text{NH}_3 \)
Concept:

The structure of a molecule can be deduced by counting the number of bond pairs and lone pairs on the central atom.

Formula:

$$ \text{Total pairs} = \text{Bond Pairs} (BP) + \text{Lone Pairs} (LP) $$

Solution:

  • The central atom shares three electrons with three surrounding atoms (3 bond pairs).


  • It has one unshared pair of electrons (1 lone pair).


  • Nitrogen in ammonia (\( \text{NH}_3 \)) has 5 valence electrons: 3 are shared with hydrogen, leaving 1 lone pair.


Why other options are incorrect:

  • Option A: \( \text{CH}_3 \) is a radical and lacks a stable octet.
  • Option B: Water (\( \text{H}_2\text{O} \)) has 2 bond pairs and 2 lone pairs.
  • Option C: Carbon monoxide (CO) involves a triple bond and a different electron arrangement.

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