Concept: Directional vs. Non-directional Bonds

Covalent bonds are directional because they involve the overlapping of atomic orbitals, which have specific orientations in space. Ionic bonds, on the other hand, are non-directional because they result from the electrostatic attraction between oppositely charged ions. This attraction is uniform in all directions around the ion.

Why (B) RbCl is correct:

Rubidium chloride (RbCl) is an ionic compound. Rubidium (Rb) is an alkali metal (Group 1) and chlorine (Cl) is a halogen (Group 17). The large electronegativity difference between Rb and Cl leads to the transfer of electrons from Rb to Cl, forming ${\mathrm{Rb}}^{+}$ and ${\mathrm{Cl}}^{-}$ ions. The electrostatic attraction between these ions is non-directional, meaning the ${\mathrm{Rb}}^{+}$ ion attracts ${\mathrm{Cl}}^{-}$ ions equally from all directions, and vice-versa.

Option Analysis:

• A) ${\mathrm{NCl}}_3$: Nitrogen trichloride is a covalent compound. Nitrogen and chlorine atoms share electrons to form covalent bonds. These covalent bonds are directional due to the specific overlap of atomic orbitals.
• B) $\mathrm{RbCl}$: Rubidium chloride is an ionic compound. The bonds are formed by electrostatic attraction between ${\mathrm{Rb}}^{+}$ and ${\mathrm{Cl}}^{-}$ ions, which is non-directional.
• C) ${\mathrm{BeCl}}_2$: Beryllium chloride is primarily a covalent compound, especially in its gaseous state. Beryllium forms two covalent bonds with chlorine atoms. These covalent bonds are directional.
• D) ${\mathrm{BCl}}_3$: Boron trichloride is a covalent compound. Boron forms three covalent bonds with chlorine atoms. These covalent bonds are directional.

Correct Answer: (B)