The Noble Gases: Occurrence and Physical Properties – Properties and Uses of Group 18 Elements (Senior 4 Chemistry) | Ganzaa

Properties and Uses of Group 18 Elements

Lessons

Senior 4 · Chemistry

Properties and Uses of Group 18 Elements

3 lessons

Lesson 1 of 3

The Noble Gases: Occurrence and Physical Properties

25 min

Nitrogen that weighed too much

In 1894 Lord Rayleigh weighed nitrogen made in two ways: by removing oxygen, carbon dioxide and water vapour from air, and by decomposing nitrogen compounds such as ammonia. The two samples should have been identical. They were not: the nitrogen from air was about 0.5% denser, every time. Rayleigh and William Ramsay at University College London then removed the nitrogen as well and were left with a small volume of a gas that would not react with anything. They called it argon, from the Greek argos, "lazy". Within four years Ramsay had found three more such gases by boiling liquid air one fraction at a time: krypton ("hidden"), neon ("new") and xenon ("stranger"). Helium had already been seen in 1868 as a yellow line in the spectrum of the Sun, before anyone found it on Earth. Rayleigh and Ramsay received Nobel Prizes in 1904 for discovering a whole new group of the periodic table.

This lesson describes where these gases are found, how they are separated, and how their physical properties change from helium to radon.

Definition

Noble gases (Group 18): helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and the artificial element oganesson (Og). Each atom has a full outer shell: helium $1 s^{2}$ , the others $ns^2\,np^6$ . Example: argon is $[\mathrm{Ne}]\,3s^2\,3p^6$ .

Monatomic: existing as single, separate atoms rather than molecules. All the noble gases are monatomic: the gas is written $\mathrm{Ar}$ , never $\mathrm{Ar_2}$ .

Fractional distillation of liquid air: separating air into its components by cooling it until it liquefies, then letting it warm in a fractionating column so that each component boils off at its own boiling point.

Where the noble gases are found

Because they hardly react, the noble gases are found uncombined, mainly in the air. Argon is the third most abundant gas in the atmosphere, after nitrogen and oxygen; the old name "rare gases" does not fit it at all.

Table 13.1: Noble gases in dry air, and their main sources

Element	Parts per million in air (by volume)	Main commercial source
Helium	5.20	natural gas wells; it forms underground from the alpha decay of uranium and thorium and collects with the gas
Neon	18.20	liquid air
Argon	9340 (0.934%)	liquid air; almost all of it is argon-40, formed by the decay of potassium-40 in rocks
Krypton	1.10	liquid air
Xenon	0.09	liquid air
Radon	traces only	radioactive decay of radium in rocks and soil; it is itself radioactive

Helium is so light that it escapes from the top of the atmosphere into space, which is why there is so little of it in air even though it is the second most abundant element in the universe.

Separating them from air

Air is filtered, dried, freed of carbon dioxide, compressed and cooled until it liquefies (below about 80 K). In the fractionating column of an air separation unit the liquid warms up and each gas boils off at its own boiling point (Figure 13.1). Argon boils between nitrogen and oxygen and is drawn off from the middle of the column; neon (with helium) stays in the gas that never liquefies; krypton and xenon collect in the liquid oxygen at the bottom and are separated from it afterwards.

Figure 13.1: Boiling points at 1 atm, on a temperature scale drawn to scale. Argon and oxygen boil only 2.9 K apart, so a tall column is needed to separate them.

Physical properties down the group

Table 13.2: Physical and atomic properties of the noble gases

Property	He	Ne	Ar	Kr	Xe	Rn
Atomic number	2	10	18	36	54	86
Electrons per shell	2	2, 8	2, 8, 8	2, 8, 18, 8	2, 8, 18, 18, 8	2, 8, 18, 32, 18, 8
Atomic radius (calculated) / pm	31	38	71	88	108	120
First ionisation energy / kJ mol⁻¹	2372	2080	1520	1351	1170	1037
Melting point / K	none at 1 atm	24.7	83.6	115.8	161.7	202.2
Boiling point / K	4.4	27.3	87.3	121.5	166.6	211.5
Enthalpy of vaporisation / kJ mol⁻¹	0.08	1.74	6.52	9.05	12.65	18.1
Density at 0 °C, 1 atm / g dm⁻³	0.1786	0.9002	1.7818	3.708	5.851	9.97
Solubility in water at 20 °C / cm³ per kg	8.61	10.5	33.6	59.4	108.1	230

Figure 13.2: Melting and boiling points of the noble gases, plotted from Table 13.2. Each liquid range is only a few kelvin wide.

All six are colourless, odourless gases at room temperature. Their trends follow from two facts: the atoms are single and spherical, and they get bigger down the group.

Boiling points, melting points and enthalpies of vaporisation rise. The only forces between the atoms are London (dispersion) forces. A larger atom has more electrons in a bigger, more easily distorted (polarisable) cloud, so the temporary dipoles are larger and the attraction stronger. Helium's forces are so weak that it boils at 4.4 K and cannot be frozen at all at normal pressure.
Atomic radius increases, because each element adds a new shell of electrons.
First ionisation energy falls, because the outer electrons are further from the nucleus and better shielded by inner shells. Even so, each noble gas has the highest ionisation energy in its period.
Density increases with molar mass. At the same temperature and pressure, equal volumes of gases hold equal numbers of moles, so density is proportional to molar mass: $\rho = \frac{M}{V_m}$ . Helium is the only noble gas less dense than air.
Solubility in water increases, again because bigger atoms are more polarisable and are attracted more strongly to the polar water molecules.

Worked example 1: Rayleigh's missing gas

"Nitrogen" from air (really nitrogen mixed with argon) was 0.5% denser than pure nitrogen made chemically. Estimate the percentage of argon by volume in that "atmospheric nitrogen". $M(\mathrm{N_2}) = 28.01\ \mathrm{g\,mol^{-1}}$ , $M(\mathrm{Ar}) = 39.95\ \mathrm{g\,mol^{-1}}$ .

Solution. At the same temperature and pressure, density is proportional to the mean molar mass, so the mixture has a mean molar mass 0.5% above that of nitrogen: $\bar{M} = 1.005 \times 28.01 = 28.15\ \mathrm{g\,mol^{-1}}$ Let $x$ be the mole (= volume) fraction of argon: $28.01(1 - x) + 39.95x = 28.15 \quad\Rightarrow\quad x = \frac{28.15 - 28.01}{39.95 - 28.01} = \frac{0.140}{11.94} = 0.0117$ About 1.2% of the "atmospheric nitrogen" was argon. Check: air has 78.08% nitrogen and 0.934% argon, and $\frac{0.934}{78.08 + 0.934} \times 100 = 1.18\%$ . A density difference in the third decimal place revealed a new element.

Worked example 2: why a helium balloon rises

Calculate the density of helium at 0 °C and 1 atm, and the upward force (expressed as the mass it can lift) on 1.00 m³ of helium in air. Molar volume $V_m = 22.41\ \mathrm{dm^3\,mol^{-1}}$ at 0 °C and 1 atm; density of air $= 1.293\ \mathrm{g\,dm^{-3}}$ ; $M(\mathrm{He}) = 4.003\ \mathrm{g\,mol^{-1}}$ .

Solution. $\rho(\mathrm{He}) = \frac{M}{V_m} = \frac{4.003}{22.41} = 0.1786\ \mathrm{g\,dm^{-3}}$ which matches the measured value in Table 13.2. 1.00 m³ = 1000 dm³, so the helium has mass 178.6 g and displaces 1293 g of air. By Archimedes' principle the net upward push equals the weight of $1293 - 179 = 1114\ \mathrm{g} \approx 1.11\ \mathrm{kg}$ For hydrogen ( $\rho = \frac{2.016}{22.41} = 0.0900\ \mathrm{g\,dm^{-3}}$ ) the lift would be 1.20 kg, so helium gives about 93% of the lift of hydrogen, but it cannot burn.

Warning

Common mistakes. (1) Explaining the rise in boiling point by "stronger bonds": the noble gases have no bonds between atoms, only London forces. (2) Writing noble gases as molecules ( $\mathrm{He_2}$ , $\mathrm{Ar_2}$ ): they are monatomic. (3) Saying every noble gas has eight outer electrons: helium has two. (4) Calling argon "rare": it makes up almost 1% of air, about 23 times as much as carbon dioxide.

Safety. Noble gases are not poisonous, but they are asphyxiants: in a closed or low-lying space a leaking cylinder of argon or helium pushes out the air, and because the gas has no smell, a person can collapse without warning. Never breathe gas from a cylinder or a balloon. Gas cylinders are stored upright and chained, and opened only by a teacher in a ventilated room. Liquid helium, nitrogen and argon cause severe cold burns: they are handled only with a face shield and insulated gloves. If someone collapses in a space where gas may have leaked, do not go in after them: raise the alarm and call for emergency help, so that you do not become a second casualty.

Tip

Down Group 18, everything to do with size goes up (radius, boiling point, density, solubility) and everything to do with holding electrons goes down (ionisation energy). The one cause behind both: one more shell of electrons per period.

FuseSchool: Noble Gases, the Gases in Group 18 (4 min). Watch for what the Group 18 gases have in common, and what makes each one different from the others.

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Practice

Write the full electron configuration of krypton and the shortened (noble-gas core) configuration of xenon.
Which noble gas is most abundant in air, and why is there so little helium in air?
Explain why argon can be separated from liquid air, but helium is obtained from natural gas.
Explain, in terms of forces between particles, why xenon boils at a higher temperature than argon.
Explain why the first ionisation energy decreases from helium to radon.
Use $\rho = \frac{M}{V_m}$ to calculate the density of xenon at 0 °C and 1 atm ( $M = 131.29\ \mathrm{g\,mol^{-1}}$ , $V_m = 22.41\ \mathrm{dm^3\,mol^{-1}}$ ), and compare it with the value in Table 13.2.
A classroom measures 5.0 m × 4.0 m × 3.0 m. Air is 0.934% argon by volume. Calculate the mass of argon in the room at 25 °C and 1 atm ( $V_m = 24.47\ \mathrm{dm^3\,mol^{-1}}$ , $M(\mathrm{Ar}) = 39.95\ \mathrm{g\,mol^{-1}}$ ).
Air contains 0.09 ppm of xenon by volume. Calculate the mass of xenon in 1.00 m³ of air at 0 °C and 1 atm, and use your answer to explain why xenon is much more expensive than argon.

Show answersHide answers

Kr: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^{10}\,4s^2\,4p^6$ ; Xe: $[\mathrm{Kr}]\,4d^{10}\,5s^2\,5p^6$ .
Argon (0.934% by volume). Helium atoms are so light and fast that they escape from the top of the atmosphere into space.
Argon makes up 0.934% of air and condenses with it, boiling at 87.3 K between nitrogen and oxygen. Helium is only 5.2 ppm of air and does not liquefy in the plant (b.p. 4.4 K); natural gas wells can hold several per cent of helium, formed by alpha decay underground.
Xenon atoms are larger with more electrons, so they are more polarisable; the temporary dipoles are bigger and the London forces between atoms stronger, so more energy is needed to separate them.
Down the group the outer electrons are in shells further from the nucleus and shielded by more inner electrons, so they are held less strongly.
$\rho = \frac{131.29}{22.41} = 5.86\ \mathrm{g\,dm^{-3}}$ , close to the measured 5.851; the small gap is because xenon is not quite an ideal gas.
Volume of room = 60 m³ = 60 000 dm³; argon = $0.00934 \times 60\,000 = 560.4\ \mathrm{dm^3}$ ; $n = \frac{560.4}{24.47} = 22.9\ \mathrm{mol}$ ; mass $= 22.9 \times 39.95 = 915\ \mathrm{g}$ , almost 1 kg.
Volume of Xe = $0.09 \times 10^{-6} \times 1000 = 9 \times 10^{-5}\ \mathrm{dm^3}$ ; $n = \frac{9 \times 10^{-5}}{22.41} = 4.0 \times 10^{-6}\ \mathrm{mol}$ ; mass $= 5.3 \times 10^{-4}\ \mathrm{g}$ (about 0.5 mg). About 100 000 times less xenon than argon is in air, so huge volumes of air must be processed for a little xenon.

Quick check

Why does xenon boil at a higher temperature than argon?

Key points

The noble gases have full outer shells ( $1 s^{2}$ for He, $ns^2\,np^6$ for the rest) and exist as single atoms.
Ne, Ar, Kr and Xe come from the fractional distillation of liquid air; helium from natural gas (made by alpha decay); radon from the decay of radium. Argon is 0.934% of air.
Down the group, atomic radius, boiling point, density and solubility increase (stronger London forces, bigger molar mass); first ionisation energy decreases.
Density of a gas at fixed T and p is proportional to its molar mass: $\rho = \frac{M}{V_m}$ .

Sources and credits

Image: Linde (previously AGA) air separation plant in Washington, West Virginia..JPG by Snoopywv, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Image: 2024-07-02 Weather Balloon Launch 019.jpg by Photouploadforscience (Indiana University South Bend), via Wikimedia Commons, licensed under CC BY 4.0.
Data and history: Noble gas and Argon, Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0, adapted by Ganzaa.
Video: Noble Gases - The Gases In Group 18, FuseSchool - Global Education, embedded from YouTube.
The text, tables, charts, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Nitrogen that weighed too much

2Where the noble gases are found

3Separating them from air

4Physical properties down the group

Nitrogen that weighed too much

Where the noble gases are found

Separating them from air

Physical properties down the group