Concept: Covalent bonds are formed by the sharing of electrons between two non-metal atoms. Ionic bonds are formed by the transfer of electrons, typically between a metal and a non-metal, resulting in electrostatic attraction between ions. Metallic bonds are found in metals, where a 'sea' of delocalized electrons holds positively charged metal ions together.

Why (A) is correct:

• ${\mathrm{BCl}}_3$: Boron (a non-metal) and Chlorine (a non-metal) form covalent bonds.
• ${\mathrm{SiCl}}_4$: Silicon (a non-metal/metalloid) and Chlorine (a non-metal) form covalent bonds.
• ${\mathrm{PCl}}_3$: Phosphorus (a non-metal) and Chlorine (a non-metal) form covalent bonds.

All molecules in option (A) consist of non-metal atoms bonded together, indicating purely covalent bonding.

Option Analysis:

• (B) ${\mathrm{NH}}_4\mathrm{Br},{\mathrm{N}}_2{\mathrm{H}}_4,\mathrm{HBr}$: ${\mathrm{NH}}_4\mathrm{Br}$ is an ionic compound (ammonium bromide) containing the polyatomic ammonium ion ${\mathrm{NH}}_4^{+}$ and bromide ion ${\mathrm{Br}}^{-}$. While there are covalent bonds within the ammonium ion, the overall compound is ionic. ${\mathrm{N}}_2{\mathrm{H}}_4$ and $\mathrm{HBr}$ are covalent.
• (C) ${\mathrm{I}}_2,{\mathrm{H}}_2\mathrm{S},\mathrm{NaI}$: $\mathrm{NaI}$ is an ionic compound (sodium iodide) formed between a metal (Na) and a non-metal (I). ${\mathrm{I}}_2$ and ${\mathrm{H}}_2\mathrm{S}$ are covalent.
• (D) $\mathrm{Al},{\mathrm{O}}_3,{\mathrm{As}}_4$: $\mathrm{Al}$ is a metal and exhibits metallic bonding. ${\mathrm{O}}_3$ (ozone) and ${\mathrm{As}}_4$ (white arsenic) are covalent molecules.

Correct Answer: (A)