Which of the following are isostructural pairs? A. SO42- and CrO42- B. SiCl4 and TiCl4 C. NH3 and NO3- D. BCl3 and BrCl3

🧠 Isostructural = same geometry (both shape and bond arrangement). Compare each pair.

A. $\ce{SO4^{2-}}$ and $\ce{CrO4^{2-}}$: Both are tetrahedral $\ce{MO4^{2-}}$ species with identical symmetry → isostructural ✓

B. $\ce{SiCl4}$ and $\ce{TiCl4}$: Both are tetrahedral $\ce{MCl4}$ species (both Si and Ti form 4 bonds, 0 LP) → isostructural ✓

C. $\ce{NH3}$ and $\ce{NO3^-}$: $\ce{NH3}$ is pyramidal (N: 3 bonds + 1 LP); $\ce{NO3-}$ is trigonal planar (N: 3 bonds + 0 LP) → not isostructural ✗

D. $\ce{BCl3}$ and $\ce{BrCl3}$: $\ce{BCl3}$ is trigonal planar (B: sp², 0 LP); $\ce{BrCl3}$ is T-shaped (Br: sp³d, 2 LP) → not isostructural ✗

$\boxed{\text{Answer: (c) A and B only}}$