问题 选择题

X、Y是短周期元素,两者组成化学式为XY2的化合物,已知X的原子序数为n,则Y的原子序数可能是

①n-3 ②n-6 ③n-8 ④n+1 ⑤n+2 ⑥n+5

A.①⑥

B.①②⑤⑥

C.①②④⑤⑥

D.全部

答案

答案:D

题目分析:两种短周期元素组成的化合物中原子数比为1:2,则化学式为AB2型,可能为第ⅡA族与ⅤⅡA组元素组成、可能为第ⅣA族和ⅥA族元素组成、还可能为NO2、SO2,注意C2H4中原子数比为1:2,然后分析原子序数的关系。

两种短周期元素组成的化合物中原子数比为1:2,则化学式为AB2型,

两种元素的原子序数分别是a和b,

①若为NO2,故④可能;

②若为SiO2,故②可能;

③若为MgCl2,故⑥可能;

④若为SO2,故③可能;

⑤若为CO2,故⑤可能;

⑥若为MgF2,故①可能。

显然①②③④⑤⑥都可能,故选D。

点评:本题考查化合物中原子序数的关系,明确常见的AB3型的化合物是解答本题的关键,题目难度较大。

连词成句
阅读理解

Every year many students went to the college bookstore to purchase the different textbooks that they need for their classes. Little do these students realize that, in doing so, they are throwing good money away. It may sound unbelievable but textbooks, in some cases, actually cost as much as, if not more than, college fees.

Many people think that college bookstores are the most convenient source for textbooks. Titles are usually kept and students may purchase new textbooks or choose to buy second-hand books and save money. But the money saved by purchasing second-hand textbooks from college bookstores is small compared to the amount they could save by shopping online.

More and more online booksellers have begun selling textbooks at heavily discounted prices. Online second-hand bookstores are another source for college books as they keep a large number of textbooks, which means sizable savings for students. Students should avoid purchasing their textbooks at the first store that they surf online since there are so many ‘unreal’ booksellers on the Net. Price isn't the only factor to consider when making an online purchase. Students should also consider shopping costs and delivery time when making their decision.

Online auctions(拍卖) are another option for students seeking a bargain. This option, however, has its pros and cons. A plus is that students can get incredibly cheap books. One problem is that while online bookstores have a system in place that ensure you receive your order within a reasonable amount of time, with an auction purchase, however, you  must depend on the auction seller's timetable.

One way that students can really save is by getting books for free. Project Gutenberg is one resource that English majors will find invaluable. Here students can legally download a huge collection of classic novels. In the past students needing help with their studies would often have to spend a lot of money buying expensive study guides. Today that is no longer the case. Cash-strapped students can now visit the website SparkNotes for free downloadable study guides as well as classics.

小题1:Which of the following is the best title of this passage?

A.Learning to Save on Textbooks

B.Why Buy Textbooks

C.Online Shopping

D.Tips for Buying Secondhand Textbooks小题2:Which of the following is NOT mentioned as a source to obtain textbooks ?

A.College bookstores.

B.Online used bookstores.

C.Online auctions.

D.Second-hand bookstore centres.小题3:The underlined expression “pros and cons” (paragraph 4) is closest in meaning to __________.

A.professionals and conservatives

B.joy and sadness

C.advantages and disadvantages

D.processes and conditions小题4:According to the passage, Project Gutenberg is probably __________.

A.a government project aimed at helping English majors

B.a project to help poor students get free learning materials

C.a website for English majors to get the guides from language experts

D.a website for students to get free sources for English learning小题5:From the passage, we can learn that__________.

A.the cost of textbooks in college is low compared to the amount spent on tuition

B.online auctions are perfect for students seeking text books

C.students should visit many websites before buying their books

D.students should consider shopping costs and delivery time above all else when making their buying decision