10. Diamond has unusual hardness and melting point. What type of solid is it? (A) ionic crystal (B) molecular crystal (C) metal crystal (D) covalent crystal

13. When an hydrogen electron makes transition from n = 2 to n = 1, which of the following statements is (are) true? I. Energy is emitted.II. Energy is absorbed. III. The electron loses energy. IV. The electron gains energy. V. The electron cannot make this transition. (A) II, III (B) V (C) I, III (D) II, III