16. The Lewis acidity of boron trihalides follows the trend: BF₃ ＜ BCl₃ ＜ BBr₃. What is the primary reason for this unexpected order? (A) The 3p and 4p orbitals of Cl and Br are too small to overlap with B. (B) Electronegativity decreases from F to Br, making B more positive in BBr₃. (C) BBr₃ has a higher bond dissociation energy than BF₃. (D) The π-bonding between the halogen and B is most effective in BF₃. (E) Steric effect in BBr₃ is lower than that in BF₃.

6. The atomic radius of aluminum (Al) is 143 pm, while that of gallium (Ga) is 122 pm. Why is Al larger than Ga, despite Ga being below Al in Group 13? (A) Ga is a transition metal, which inherently has a smaller radius than p-block elements like Al. (B) The addition of the 3d subshell in Ga provides poor shielding, leading to a higher effective nuclear charge that pulls valence electrons closer. (C) Al has more electron shells occupied than Ga, causing its atomic radius to be larger. (D) Ga has a more stable electron configuration that allows its valence shell to contract more easily than Al's. (E) Ga is larger than Al.

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