The molecular world
The molecular world

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

The molecular world

4.5.2 Noble gas configurations under stress

It is remarkable how many molecules and ions of the typical elements can be represented by Lewis structures in which each atom has a noble gas shell structure. Nevertheless, many exceptions exist. According to the periodic trends summarised in Section 2, the highest fluorides of boron and phosphorus are BF3 and PF5. However, phosphorus, in accordance with Table 1, also forms the lower fluoride PF3. All three compounds are colourless gases at room temperature and contain the molecules BF3, PF3 and PF5. As the valency of fluorine is one (Table 1), each bond in these molecules is a shared electron pair, and we may write the Lewis structures as follows:

Question 26

In how many of these Lewis structures do all the atoms have noble gas shell structures?

Answer

In only one, namely that of PF3; in Structure 5.11, the three shared electron pairs around the boron atom are two electrons short of the shell structure of neon, and in Structure 5.13, the five electron pairs around the phosphorus atom give us two electrons more than the shell structure of argon.

You will meet more exceptions of this sort in Section 6. Here, we merely note their existence, and observe that they are a consequence of our assumption that a chemical bond consists of two electrons shared between two atoms.

Skip Your course resources
S205_2

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus