Fill in the blanks with given words.（读句子，用所给的单词填空）     

either    sometimes    weekend    in    

usually    about       together    before 

1. What do you do on the __________________?

2. I _______________  do morning exercises at 8:30.

3. I go to work at 9:00 ___________________ the morning.

4. Thank you for telling me __________________  your day.

5. I often play football ___________________ I go hiking.

6. It's going to rain. I can't go hiking. I can't play football, ___________________.

7. You should clean your hands _____________________ meals.

8. Let's play football ____________________ next weekend.

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____________________；