Chemistry - CHEMICAL BONDING Question with Solution | TestHub

The octet rule states that atoms tend to gain, lose, or share electrons to achieve eight electrons in their valence shell.

Let's analyze each option:

A. ${\underline{\mathrm{BI}}}_3$ (Boron triiodide):

Boron (B) is in Group 13 and has 3 valence electrons. It forms 3 single bonds with iodine atoms.

Number of electrons around B = $3\times 2=6$.

Boron has an incomplete octet (6 electrons).

B. ${\underline{\mathrm{BeF}}}_3{}^{-}$ (Trifluoroberyllate ion):

Beryllium (Be) is in Group 2 and has 2 valence electrons. In ${\mathrm{BeF}}_3{}^{-}$, Be forms 3 single bonds with fluorine atoms and carries a negative charge.

Number of electrons around Be = $3\times 2=6$.

Beryllium has an incomplete octet (6 electrons).

C. ${\underline{\mathrm{ClF}}}_3$ (Chlorine trifluoride):

Chlorine (Cl) is in Group 17 and has 7 valence electrons. It forms 3 single bonds with fluorine atoms.

Number of electrons involved in bonding = $3\times 2=6$.

Number of lone pair electrons = $7-3=4$ (2 lone pairs).

Total electrons around Cl = $6+4=10$.

Chlorine has an expanded octet (10 electrons).

D. $\underline{\mathrm{CO}}$ (Carbon monoxide):

Carbon (C) is in Group 14 and has 4 valence electrons. Oxygen (O) is in Group 16 and has 6 valence electrons.

In CO, there is a triple bond between C and O.

Number of electrons around C = $3\times 2$ (from triple bond) + 2 (from one lone pair on C) = 8.

Number of electrons around O = $3\times 2$ (from triple bond) + 2 (from one lone pair on O) = 8.

Both carbon and oxygen follow the octet rule.

Final Answer: The final answer is $\boxed{D}$.