06. The impossible compounds

What made some elements so difficult to discover?

Read the following passage slowly and carefully, if you are stuck with any of the words/language, ask for assistance.

Over 300 years ago, Isaac Newton had shown that white light was a mixture of all the different colours of the rainbow. Scientists had developed this idea into the spectroscope, which allowed them to look in great detail at the spectrum of light from different sources. When some elements are heated, they give off particular colours of light. One example is sodium, which gives the orange colour that you see in street lights. These colours show up as lines in the spectrum. Each element has a different set of lines, and the pattern of lines acts like a ‘bar code’ to identify the element.

In 1868, during an eclipse of the Sun, Pierre Janssen (1824–1907) found a line in the spectrum of sunlight that did not belong to any known element. He decided that this must be a new element, which was named helium after the Greek word for the Sun, helios. Just over 100 years ago, in 1895, helium was discovered on Earth. It is found in tiny amounts in the atmosphere, and also in natural gas. At about the same time, a whole family of gases like helium was discovered: argon, neon, krypton and xenon. These gases shared one remarkable property that had made them so difficult to find. They did not take part in any chemical reactions at all! Because of this, they were known as the inert gases.

In 1916, Gilbert Lewis (1875–1946) came up with a theory of electron shells, which explained why atoms bonded together and why the inert gases didn’t react. This theory seemed to work well, and for over 40 years scientists assumed that reactions of the inert gases were impossible.

In 1962, Neil Bartlett (born 1932) was carrying out research with platinum hexafluoride, PtF₆. He knew that platinum hexafluoride was a gas that reacted very easily, even with substances which didn’t normally react. He thought that the larger the atom, the better the chance of a reaction. Maybe Lewis’s theory was wrong and the xenon would react after all.

When he mixed the two gases together, they reacted straight away, and an orange solid was formed. He had made the first ever compound of xenon. Within a year, other compounds had been made, including three different fluorides XeF₂, XeF₄ and XeF₆, and an oxide XeO₃ This was followed by the making of krypton difluoride, KrF₂

Because the gases do have a few reactions, they are often known these days as the noble gases rather than the inert gases. In the same way that the ‘nobles’ didn’t mix with the common people, the noble gases don’t react much with the other elements. Helium, neon and argon have still not been made to react with any other elements at all. These chemical reactions really are impossible

... or are they?

Activity - Answer the following questions using full English sentences were appropriate

1. Helium was discovered in the Sun in 1868. How many years later was it that:

a helium was discovered on Earth

b the first compound of xenon was made?

2. Why did it take so long to find the noble gases in the atmosphere on Earth?

3. Between the 1920s and the 1950s, scientists didn’t even bother trying to make any compounds of the noble gases. Why was this?

4. What evidence did Neil Bartlett have to suggest that it might be worth trying to make some noble gas compounds after all?

5. Which of these statements best describes Lewis’s theory after Bartlett’s experiments?

A The theory has worked for over 40 years so Bartlett’s experiments must be wrong.

B The theory needs to be changed now that we have new evidence.

C The theory is completely wrong and needs to be scrapped.

Explain your answer using evidence from the passage.

6. The compound XeF₄ is called xenon tetrafluoride. From the passage, find the chemical formula of:

a xenon difluoride

b xenon hexafluoride.

7. From the passage, find an example of:

a a scientist making observations

b a scientist drawing a conclusion

c a scientist making a prediction.

8. Imagine that in the future we do make a compound of argon. Choose two of these ways of communicating the discovery.

a Imagine that you are the scientist who has made the compound, and write an e-mail to a friend telling him or her about it,

b Write a tabloid newspaper report on the new discovery.

c Make an audio report for radio news.

d Make a video report for a popular science programme on TV.

e Produce a computer slide presentation as if you were presenting your results to a scientific conference.

Can you...

> obtain relevant information from text

> communicate scientific information clearly

> explain why theories change with time