Chemistry

Key unit competence: Compare and contrast the properties of the group 18 elements in relation to their position in the periodic table.

Learning objectives

By the end of this unit, I will be able to:

•State the physical properties of the Group 18 elements.

•Explain the lack of reactivity of the group 18 elements.

•Associate chemical inertia of the group 18 elements to their full valence shell.

•Recognize the importance of noble gases or group 18 elements in the daily life.

Introductory activity

Make a research to find out the type gas :Inside the Bulb, in balloon, responsible for different colors dispayed by this house (or in advertising sings)

Group 18 consists of six elements: helium(2He), neon(10Ne), argon(18Ar), krypton(36Kr), xenon(54Xe) and radon( 86Rn).They are all gases and are characterized by filled s and p valence orbitals (valence electron configuration of 2s2 for helium and ns2np6 for the others).This structure has been qualified as ”the octet electronic structure”; octet means eight. This electronic structure, also called octet rule gives stability to the elements of Group 18.

Because of this, these elements are very unreactive. They form very few compounds. Because of this they are termed noble gases.

Other chemical elements tend to react, to combine with other elements or with themselves in order to fulfill the octet rule, i.e. to acquire the same electronic structure as the nearest noble gas.

13.1. Occurrence and physical properties of noble gases

Activity 13.1

The air is composed of a mixture of gases including water vapour.

i)Make a research (with any documentation) to identify its components and ar-range them according to their abundances (Component1> Component 2, etc...)

ii)Show how these components can react each other if possible •If not possible, justify your answer.

iii)Explain how neon lamp works.

13.1.1. Occurrence

•All the noble gases except radon occur in the atmosphere. Their total atmospheric abundance in air is 0.03%; argon is the major component.

• Helium and sometimes neon are found in minerals of radioactive origin e.g., pitchblende, monazite, cleveite.

.Xenon and radon are the rarest elements of the group. Radon is obtained as a decay product of 226Ra.

13.1.2. Physical properties

•All the noble gases are monoatomic.

•They are colourless, odourless and tasteless.

•They are sparingly soluble in water.

•They have very low melting and boiling points.

•Due to stable electronic configuration these gases exhibit very high ionisation energy. The ionization energy decreases down the group with increase in atomic size.

•Since they have stable electronic configurations, they have low tendency to accept or give up electron.

Helium has the lowest boiling point of any known substance: ~ 267 – 2690C

Checking up 13.1

Question:

(i) Using dot and cross diagram, show how neon(z =10) is considered as a noble gas.

(ii) Why do we classify helium in Group 18?

13.2. Reactivity and inertness of noble gases

Activity 13.2

1. Through a research, can you find any compound made of one of the rare gas?

2. Do you think that noble gases can react with one of these reagents in normal conditions?

Although group 18 elements used to be called inert gases, it has been discovered that some of them, in appropriate conditions can react and form compounds.

But up to now, of the group 18 elements, only krypton and xenon have been shown to form chemical compounds. The first of these was prepared in 1962 by Neil Bartlett,

an English chemist who made the ionic compound XePtF6, which contains the ions Xe+ and PtF6-:

It has also been discovered that xenon tetrafluoride can be prepared by reacting xenon and fluorine gas in a nickel reaction vessel at 400oC and 6 atmospheres:

Xenon tetrafluoride forms stable colorless crystals. Two other xenon fluorides, XeF2and XeF6, were synthesized by the group of Argonne, and a highly explosive xenon oxide (XeO3) was also found. The xenon fluorides (XeF2, XeF4 and XeF6 ) are colourless crystalline solids and sublime readily at about 25oC. They are powerful fluorinating agents.

They are readily hydrolysed even by traces of water, for example:

In the past years other xenon compounds have been prepared, for example, XeO4(explosive), XeOF4, XeOF2, and XeO3F2. These compounds contain discrete molecules with covalent bonds between the xenon and the other atoms. A few compounds of krypton, such as KrF2 and KrF4, have also been observed. There is evidence that radon also reacts with fluorine, but the radioactivity of radon makes its chemistry very difficult to study.

Checking up 13.2

Question: Explain why in some applications such as air balloons, helium preferred to hydrogen?

13.3. Uses of noble gases

Activity: 13.3

Do a research (with any documentation) to find how each noble gas has been discovered and its uses?

Helium

•Helium is a non-inflammable and light gas. Hence, it is used in filling balloons for meteorological observations, replacing the flammable hydrogen gas.

•It is also used in gas-cooled nuclear reactors.

•Liquid helium (B.P:-267.8oC) finds use as cryogenic agent for carrying out various experiments and conservation at very low temperatures.

Neon

•Neon is used in advertising signs, it glows when electricity is passed through it. Different coloured neon lights can be made by coating the inside of the glass tubes with colored chemicals.

•Neon bulbs are more used in our daily life.

Argon

•It is used in light bulbs. The very thin metal filament inside the bulb would react with oxygen and burn away if the bulb were filled with air instead of argon.

•Argon is used mainly to provide an inert atmosphere in high temperature metallurgical processes (arc welding of metals or alloys).

•It is also used in the laboratory for handling substances that are air-sensitive.

Krypton

Krypton is used in lasers. Krypton lasers are used by surgeons to treat certain eye problems and others. It is used in light bulbs designed for special purposes.

Xenon

Xenon is used in fluorescent bulbs, flash bulbs and lasers. Xenon emits an instant, intense light when present in discharge tubes. This property of xenon is utilized in high-speed electronic flash bulbs used by photographers.

Radon

Radon is radioactive and is used in medicine as a source of gamma rays. The gas is sealed in small capsules, which are implanted in the body to destroy malignant (e.g., cancerous) growths.

13.4. End unit assessment

1. a) Give a reason why the first ionization energies of noble gases are very high.

b) State one use of neon and give a reason to support your answer.

c) State and explain the trend in atomic radius among noble gases.

d) Why are noble gases unreactive?

e) Explain why the value of the first ionisation energy of neon is higher than that of sodium.

2. Explain why Group 18 elements are rare on Earth?

3. The discovery of compounds of noble gases has been done, up to date, with Xe and Kr, not with He or Ne. Can you suggest a probable reason?