Dear Maggie,

I’ve just realized I haven’t written to you for a month. How have you done with yourself since I last see you? In your last letter you said you were thinking of ask for a new job. I’d love to hear what was happened. I’d have phoned, but I may have lost your number. I remember writing it up somewhere and I haven’t been able to find them. Anyway, why I’m writing is tell you I’m having a party next Friday. Whether you’d like to come, I suggest you give me a ring and I’ll give you all the details. I hope you’ll be able to come and looking forward to hearing from you.

写出下列化学符号或符号的意义：

（1）氧元素　  　；（2）氦气　   　；（3）三个钠离子　   　；

（4）氧化镁中镁元素的化合价为+2价　　；

（5）2H　     　；（6）3CO₂　     　；