Question

A molecule having an incomplete octet in the central atom is

• A. ${\mathrm{H}}_2\mathrm{O}$
• B. ${\mathrm{NH}}_3$
• C. ${\mathrm{PCl}}_5$
• D. ${\mathrm{BCl}}_3$

Answer 1

The correct option is D

${\mathrm{BCl}}_3$

Explanation for correct option:

Option (d) ${\mathrm{BCl}}_3$

• The Octet rule refers to atoms' preference for having eight electrons in their valence shell.
• Atoms with fewer than eight electrons are more likely to react and produce more stable compounds.
• The octet rule does not apply to ${\mathrm{BCl}}_3$. It is an electron-deficient molecule.
• Because it only shares three electrons with the Chlorine ($\mathrm{Cl}$) atom.
• Boron ($\mathrm{B}$) possesses just six electrons after creating a molecule, three from the Chlorine atom and three from itself.

Explanation for incorrect options:

Option (a) ${\mathrm{H}}_2\mathrm{O}$

• Water (${\mathrm{H}}_2\mathrm{O}$) molecules follow the Octet rule and have form complete octet as shown below:

• So, this option is incorrect.

Option (b) ${\mathrm{NH}}_3$

• Ammonia (${\mathrm{NH}}_3$) also has a complete octet with six shared electrons and two electrons from lone pair.

• So, this option is also incorrect.

Option (c) ${\mathrm{PCl}}_5$

• Phosphorus pentachloride (${\mathrm{PCl}}_5$) also has a complete octet with five Chlorine ($\mathrm{Cl}$) atoms surrounding the Phosphrous atom as shown below :

• So, this option is also not correct.

Hence, the molecule having an incomplete octet in the central atom is ${\mathrm{BCl}}_3$, option (d).