18. The complex [Co(en)₂Cl₂]⁺ exists as several isomers. Which of the following statements is correct regarding its stereochemistry? (A) The cis isomer is a meso compound and is optically inactive. (B) The cis isomer is chiral and exists as a pair of enantiomers (Δ and Λ). (C) The trans isomer is chiral and has a non-superimposable mirror image. (D) The complex can only exist as structural isomers, not stereoisomers. (E) Both the cis and trans isomers are optically active.

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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