(三)元素周期表中主族元素性质的递变规律

(二)实质：元素原子核外电子排布的周期性变化。

(一)定义：元素的性质随着核电荷数的递增而呈周期性变化的规律。

(二)原子核外电子排布规律

1、核外电子一般总是尽先排布在能量最低的电子层里。

2、每个电子层最多容纳的电子数为2n²个。

3、最外层最多容纳电子数不超过8个(K层为最外层时不超过2个)。

4、次外层最多容纳的电子数目不超过18个，倒数第三层不超过32个。

(一)在同一原子中各电子层之间的关系

20. Mrs. White became a teacher in 1985.She for twenty years by next summer. 　

A. will teach　 　　　　　　　　　　　　　　　　　　　　 B. would have taught

C. has been teaching　 　　　　　　　　　　　　 D. will have been teaching

19. No decision about any future appointment until all the candidates have been interviewed.

A. will be made　　　　　　　　　　　　　　　　　　　　　　 B. is made

C. is being made　　　　　　　　　　　　　　　　　　　　　 D. has been made

18. --- You’ve sent the report to the newspaper office, haven’t you?

　 --- Not yet. It _____ by 12 o’clock.

A. has to be typed 　　　　　　　　　　　　　　　　　　　 B. will be typed　　　　　 C. will have been typed　　 D. needs typing

17.Over the past 30 years，China significant changes in comprehensive national strength thanks to the reform and opening-up policy.

A. witnessed 　　　　　　　　　　　　　　　　　　　　　　　　 B. were witnessing

C. has witnessed　 　　　　　　　　　　　　　　　　　　　　 D. will have witnessed

16.--Why don’t you buy the necklace since you like it so much?

　 --I____, but I don’t have the money.

A. would 　　　　　　　　 B. will　　　　　　　　　　　　　　　 C. should　 　　　　　　　　 D. may