Chemistry 2e · Representative Metals, Metalloids, and Nonmetals
Occurrence, Preparation, and Properties of the Noble Gases
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The noble gases — helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and Radon Radioactive noble gas (Rn-222) from uranium-bearing soil and rock Full entry → (Rn) — make up Group 18. Each has a completely filled valence shell (ns2np6, or 1s2 for helium), which is why they exist as colorless, odorless, monatomic gases that are strikingly unreactive. "Noble" means they keep to themselves — but not absolutely: since 1962 chemists have made many xenon, krypton, and radon compounds with fluorine and oxygen. This topic covers where each gas comes from, how industry isolates them from air and natural gas, and the technologies that depend on them, from MRI magnets to neon signs.
Why this matters
- Industry: Argon shields welds and fills light bulbs; helium cools MRI magnets; neon, krypton, and xenon power signs, windows, and lamps.
- Safety: Radon, a radioactive gas seeping from uranium-bearing soil into basements, is a leading cause of lung cancer; testing and mitigation save lives.
- Chemistry: Group 18 is the clearest illustration of how electron configuration and Ionization energy Energy needed to remove one electron from a gaseous atom Full entry → control chemical behavior.
- Exams: Configurations, the ionization-energy trend, Fractional distillation Separating a liquid mixture by boiling components off one at a time by boiling point Full entry → of air, and xenon's compounds are reliable questions.
The college version
Core Concepts
Electron configurations and the trend down the group
Configurations: He 1s2; Ne [He]2s22p6; Ar [Ne]3s23p6; Kr [Ar]4s23d104p6; Xe [Kr]5s24d105p6; Rn [Xe]6s24f145d106p6. Atomic radius increases down the group. First ionization energy decreases — from helium's record 2372 kJ/mol to radon's ~1037 kJ/mol — because outer electrons sit farther from the nucleus and are better shielded. This trend explains why xenon, not helium, was the first Noble gas A Group 18 element with a completely filled valence shell Full entry → forced into stable compounds.
Occurrence: air, natural gas, and radioactive decay
- Argon is the third most abundant gas in dry air at ~0.93% by volume — the most plentiful noble gas on Earth. Neon, helium, krypton, and xenon are trace components of air.
- Helium: terrestrial helium comes from Alpha decay Radioactive process ejecting an α particle (42He2+) Full entry → — the α particle ejected by uranium-238 and thorium-232 decay chains is a helium nucleus (42He2+). It accumulates in natural gas fields (up to ~7% by volume), the commercial source.
- Radon is generated continuously by decay of radium-226 (uranium-238 series) in rocks and soil. Radon-222 has a 3.8-day half-life and can build up indoors through foundation cracks.
Preparation: fractional distillation of liquid air
Air is the industrial source of Ar, Ne, Kr, and Xe. Filter the air, remove water vapor and CO₂, compress and cool until it liquefies, then fractionally distill: nitrogen (bp –196 °C) boils off first, then argon (–186 °C), then oxygen (–183 °C). Krypton and xenon concentrate in the least volatile residue; neon and helium stay with the nitrogen fraction and are purified by low-temperature adsorption. Helium from natural gas is recovered cryogenically: everything else condenses while helium (bp 4.2 K, the lowest of any substance) stays a vapor.
Properties: physical behavior and the limits of "inertness"
All noble gases are monatomic and nonpolar; only weak London dispersion forces act between atoms, so boiling points are extremely low but rise down the group (He 4.2 K → Ne 27 K → Ar 87 K → Kr 120 K → Xe 165 K → Rn 211 K). Closed shells mean no normal bonding, but the ionization-energy trend lets heavier members react with fluorine and oxygen. In 1962 Neil Bartlett made the first noble gas compound, XePtF₆; xenon also forms XeF₂, XeF₄, XeF₆ and oxides XeO₃/XeO₄, and krypton forms KrF₂ under vigorous conditions. Radon, with the lowest ionization energy, is the most reactive of the group (RnF₂).
Uses that depend on the properties
- Helium: less dense than air (Worked Example 1) → balloons and airships; nonflammable heliox mix for divers; liquid He cools MRI magnets.
- Argon: cheap inert shielding gas for arc welding and for filling bulbs (protects the tungsten filament).
- Neon: glows red-orange when an electric discharge passes through it (other sign colors come from other gases or phosphors).
- Krypton/xenon: krypton fills high-efficiency windows; xenon drives flash lamps and headlights and is an inhalational anesthetic.
- Radon: a hazard, not a product — its inhaled decay products cause lung cancer (second only to smoking in many public-health summaries). Response: test kits, ventilation, sealing of entry routes.
How It Works / Step-by-Step Process
Predicting whether a noble gas forms compounds:
- Write the configuration; confirm the valence shell is full.
- Compare ionization energy with lighter members: lower IE → bonding with F or O becomes possible.
- Apply the trend: He/Ne/Ar — no stable compounds; Kr — only KrF₂; Xe — fluorides, oxides; Rn — most reactive.
Isolating argon from air:
- Filter air; remove water vapor and CO₂.
- Compress and cool until the air liquefies.
- Fractionally distill; collect the argon cut boiling off between N₂ and O₂ (–186 °C).
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| "Noble gases never react" | "Noble gases are very unreactive" | Xe, Kr, Rn form stable compounds with F and O; only He, Ne, Ar are effectively inert |
| Helium (nonflammable) | Hydrogen (flammable) | Hindenburg-era airships burned because they used H₂; He is safe — a classic exam trap |
| "Neon signs contain neon" | Most "neon" signs use other gases | Only red-orange tubes are neon; other colors come from argon, mercury, phosphors |
| "Radon pollution comes from industry" | Radon comes from the ground | Rn forms by radioactive decay in soil and rock and enters through foundations |
| Argon is rare | Argon is scarce | Ar is 0.93% of dry air — the most abundant noble gas on Earth |
Eli explains
The same idea, in plain words
Explain it like I’m 10
These gases have completely full electron shells, so they don't want to trade, share, or steal electrons — we call them "noble." That laziness is useful: helium keeps balloons up and cools big magnets, argon protects hot metal while it's welded, and neon signs glow when electricity excites these lonely atoms. The biggest ones (xenon, radon) can be forced to react with fluorine — and radon is radioactive, so it can be dangerous in basements.
Worked example
Example 1: Why does helium float? Density of He vs. air at STP
Gas density at STP (0 °C, 1 atm) comes from the ideal gas law rearranged:
d = PMRT
where P is pressure (atm), M molar mass (g/mol), R = 0.08206 L·atm·mol-1K-1, T temperature (K). For helium, M = 4.003 g/mol:
dHe = (1 atm)(4.003 g/mol)(0.08206 L·atm·mol-1K-1)(273.15 K) = 0.179 g/L
Dry air has an effective molar mass of about 28.97 g/mol:
dair = (1 atm)(28.97 g/mol)(0.08206 L·atm·mol-1K-1)(273.15 K) = 1.292 g/L
Answer: Each liter of helium is 1.292 - 0.179 = 1.11 g lighter than a liter of air, so a helium balloon lifts about 1.1 g per liter.
Example 2: How much argon is in the air around you?
A classroom holds 4.5 × 105 L of air at STP. Argon is 0.93% of dry air by volume. How many moles of argon are present?
Formula first: volume of Ar = (volume fraction) × (total volume).
VAr = 0.0093 × 4.5 × 105 L = 4.2 × 103 L
Convert to moles with the molar volume at STP, 22.4 L/mol:
nAr = 4.2 × 103 L × 1 mol22.4 L = 1.9 × 102 mol
Answer: About 1.9 × 102 mol — roughly 7.5 kg of argon.
Key takeaways
- Group 18 = noble gases; closed ns2np6 shells (He 1s2); colorless, odorless, monatomic gases.
- Very unreactive, but not inert: Xe, Kr, and Rn form compounds with F and O; first was XePtF₆ (Bartlett, 1962).
- Ionization energy decreases down the group (He highest of all elements, 2372 kJ/mol) — why heavier noble gases react.
- Boiling points rise down the group; helium's 4.2 K is the lowest of any substance.
- Argon is ~0.93% of dry air; He comes from natural gas (alpha decay of U/Th); Rn is a decay product of radium-226.
- Preparation: fractional distillation of liquid air (Ar/Ne/Kr/Xe); cryogenic separation (He).
- Uses: He (balloons, MRI cryogenics), Ar (welding, bulbs), Ne (signs), Xe (lamps, anesthesia), Rn (hazard — test basements). Gas density at STP: d = PMRT.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Why are the noble gases so unreactive, and why does reactivity increase slightly down the group?
Show answer
Closed valence shells give very high ionization energies and no tendency to bond. Down the group, ionization energy falls (He 2372 → Rn ~1037 kJ/mol), so heavier atoms can react with F and O.
Which noble gas is most abundant in the atmosphere, and how is it obtained industrially?
Show answer
Argon (~0.93% by volume), separated from liquid air by fractional distillation between nitrogen and oxygen.
Why does xenon form stable compounds with fluorine but helium does not?
Show answer
Xenon's ionization energy is low enough for fluorine to bond with it; helium's 2372 kJ/mol is far too high.
Where does commercial helium come from, and what nuclear process created it?
Show answer
From natural gas fields, where helium accumulated over geologic time as a product of alpha decay of uranium-238 and thorium-232.
Calculate the density of argon at STP using d = PM/RT (M = 39.95 g/mol).
Show answer
d = (1)(39.95)/[(0.08206)(273.15)] = 1.78 g/L.
Why is radon a concern in homes, and what is the practical response?
Show answer
Rn-222 is a radioactive gas from the uranium-238 decay chain; it seeps from soil into basements, and its inhaled decay products are a major lung-cancer risk. Response: test, then ventilate and seal entry routes.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Noble gas
- A Group 18 element with a completely filled valence shell
- Fractional distillation
- Separating a liquid mixture by boiling components off one at a time by boiling point
- Alpha decay
- Radioactive process ejecting an α particle (42He2+)
- Ionization energy
- Energy needed to remove one electron from a gaseous atom
- Radon
- Radioactive noble gas (Rn-222) from uranium-bearing soil and rock
Sources & references
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