Argon

Argon (Ar) is the most abundant noble gas in the atmosphere (~0.9%). Its complete inertness makes it useful wherever an unreactive atmosphere is needed.

Where argon appears in 0620

Argon carries marks in two parts of the syllabus, and never through an equation. In The Periodic Table it is the worked example for why Group VIII (0) gases are unreactive and exist as single atoms. In Chemistry of the environment it is the “noble gases (~1%)” slice of clean dry air, and it turns up in questions on the fractional distillation of liquid air. Every mark comes from explaining behaviour, not from reactions.

Position in the periodic table

• Symbol: Ar
• Atomic number: 18
• Group 18 (0/VIII), Period 3
• Electron configuration: 2, 8, 8
• Monatomic: exists as single atoms (Ar), not molecules

Physical properties

Property	Detail
State at room temperature	Gas
Colour	Colourless
Smell	Odourless
Boiling point	-186 C
Density	Denser than air (Ar = 40, air ~29)
Reactivity	Completely unreactive (inert)

Why noble gases are unreactive

Noble gases have full outer electron shells:

• Helium: 2
• Neon: 2, 8
• Argon: 2, 8, 8

A full outer shell is a very stable electron configuration. Noble gas atoms have no tendency to:

• Lose electrons (no metallic bonding)
• Gain electrons (no ionic bonding)
• Share electrons (no covalent bonding)

This is why they exist as individual atoms (monatomic) and do not form compounds.

Argon in the atmosphere

Argon makes up approximately 0.9% of the atmosphere, the third most abundant gas after nitrogen (78%) and oxygen (21%). It is separated from air by fractional distillation of the liquid: air is cooled to about -200 C, then warmed slowly. Nitrogen (bp -196 C) boils off first, argon (bp -186 C) next, and oxygen (bp -183 C) last, so argon is drawn off between the other two.

Uses

Use	Reason
Welding shield gas	Inert atmosphere prevents oxidation of hot metals
Filament light bulbs	Inert, prevents filament oxidising/burning
Double-glazed windows	Poor thermal conductor, reduces heat transfer
Laboratory inert atmosphere	Prevents unwanted reactions

All uses depend on argon’s inertness and on it being cheap (the most abundant noble gas).

Noble gas trends

Property	He	Ne	Ar	Kr
Atomic number	2	10	18	36
Boiling point (C)	-269	-246	-186	-152
Density	Very low	Low	Moderate	Higher

Boiling points increase down the group because the atoms are larger, with stronger intermolecular forces (van der Waals forces) to overcome.

Key facts

• Symbol: Ar — proton number: 18
• Position: Group VIII (0), Period 3
• Electron configuration: 2, 8, 8 (full outer shell)
• Key property: monatomic and chemically inert
• Abundance: ~0.9% of dry air, the third most abundant gas
• Main use: inert shielding gas for welding and filling filament lamps

Argon vs other noble gases

• Helium is used where very low density is needed (balloons, airships) — see helium
• Neon glows red-orange in advertising signs when electricity passes through it
• Argon is preferred for welding and light bulbs because it is abundant and therefore inexpensive

Common exam mistakes

• Writing the formula as “Ar2”. Argon is monatomic, so its formula is simply Ar — it never pairs up into molecules.
• Explaining inertness by “it is a gas”. The reason is the full outer shell (2, 8, 8): a stable configuration with no tendency to gain, lose or share electrons.
• Claiming every noble gas has eight outer electrons. Helium has only two, and a full first shell of two is still a full shell.
• Swapping the uses of argon and neon. Neon glows in signs; argon fills filament bulbs and shields welds.

Worked exam questions

Argon is sealed inside a filament light bulb instead of air. Explain why. (2 marks)

Mark scheme

• Argon is inert / unreactive / has a full outer electron shell [1]
• It stops the hot tungsten filament reacting with the oxygen in air, which would make it burn out / oxidise [1]

Examiner note: “It is a noble gas” on its own scores nothing. Link the inertness to a full outer shell, then state clearly what the argon prevents.

The boiling points of three noble gases are: helium -269 C, neon -246 C, argon -186 C. Predict and explain the trend in boiling point down Group VIII. (3 marks)

Mark scheme

• Boiling point increases down the group [1]
• Atoms get larger / have more electrons, so the intermolecular (van der Waals) forces between atoms are stronger [1]
• More energy is needed to separate the atoms, so the boiling point is higher [1]

Examiner note: the forces act between whole atoms, not inside them. Never write “the bonds break” — there are no bonds between noble gas atoms.