13. In the solid state, why does cesium chloride (CsCl) adopt a coordination number of 8 while sodium chloride (NaCl) adopts a coordination number of 6? (A) The lattice energy of NaCl is higher than that of CsCl. (B) Cesium is more electronegative than sodium. (C) NaCl forms a body-centered lattice while CsCl forms a face-centered lattice. (D) The larger size of the Cs⁺ cation allows more Cl⁻ ions to surround it without anion-anion repulsion. (E) NaCl is more covalent in nature than CsCl.

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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