Concept: The octet rule states that atoms tend to form bonds in such a way that they acquire eight electrons in their valence shell, achieving a stable electron configuration similar to that of noble gases. For the central atom in a molecule, we count the number of electrons involved in bonding (shared pairs) and any lone pairs to determine if it has a complete octet (8 electrons).

Why (D) is correct:

• In ${\mathrm{CO}}_2$, carbon is the central atom. Carbon forms two double bonds with two oxygen atoms.
• Each double bond consists of 4 shared electrons. Since there are two double bonds, the carbon atom shares a total of $4\times 2=8$ electrons.
• Therefore, the central carbon atom in ${\mathrm{CO}}_2$ has a complete octet.

Option Analysis:

• A) $\mathrm{LiCl}$: Lithium (Li) is a metal and forms an ionic bond with chlorine (Cl). In $\mathrm{LiCl}$, Li loses an electron to become ${\mathrm{Li}}^{+}$ (duplet, not octet) and Cl gains an electron to become ${\mathrm{Cl}}^{-}$ (octet). The question asks for the central atom, but LiCl is a diatomic molecule, and Li does not achieve an octet.
• B) ${\mathrm{BeH}}_2$: Beryllium (Be) is the central atom. It forms two single bonds with two hydrogen atoms. Each single bond contributes 2 electrons to Be. So, Be has $2\times 2=4$ electrons in its valence shell, which is an incomplete octet.
• C) ${\mathrm{BCl}}_3$: Boron (B) is the central atom. It forms three single bonds with three chlorine atoms. Each single bond contributes 2 electrons to B. So, B has $3\times 2=6$ electrons in its valence shell, which is an incomplete octet.

Correct Answer: (D)