A:①③属于不同种元素
B:②表示的元素在化合物中通常显+1价
C:②③的化学性质相似
D:④属于离子,离子符号为Mg
⑴You need a swimming cap. ____
⑵First,you should wash yourself. ____
⑶Next,you should do warm-up exercises. ____
⑷Then, you can enjoy your swimming lessons. ____
⑸You need a swimsuit.____
A. B. C. D. E.
②2C(s)+O2(g)═2CO(g)△H=-220kJ•mol−1
H-H、O=O和O-H键的键能分别为436 kJ·mol-1、496 kJ·mol-1和462kJ•mol−1,则a为( )