15. Which of the following species has the smallest radius? (A) Se²⁻ (B) Sr²⁺ (C) Br⁻ (D) Kr (E) Rb⁺

18. Most valence electron transitions occur in the _ region of the electromagnetic spectrum. Rotational transitions are typically induced by radiation in the region, and vibrational transitions are typically induced by radiation in the _ region. (A) X-ray, UV-vis, IR (B) UV-vis, IR, microwave (C) IR, microwave, UV-vis (D) Microwave, X-ray, UV-vis (E) UV-vis, microwave, IR