20. In the molecular orbital diagram of an octahedral complex, how does π-backbonding from the metal to a ligand like CO affect the value of Δoct? (A) It increases Δoct by increasing the repulsion between ligands. (B) It increases Δoct by lowering the energy of the t₂g orbitals. (C) It has no effect on Δoct as π-bonding only involves eg* orbitals. (D) It decreases Δoct by lowering the energy of the eg* orbitals. (E) It decreases Δoct by raising the energy of the t₂g orbitals.

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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