Helium

Helium (He) is the lightest noble gas and the second lightest element after hydrogen. Its unusual pairing of low density and total inertness gives it distinctive uses.

Where it appears in 0620

Helium earns marks as a concept element rather than a reactions element — it has no reactions to learn:

• Noble gases: helium is the first member, used to introduce full outer shells, inertness, monatomic gases, and the boiling-point trend down Group 0.
• Atomic structure: helium shows that the first electron shell is full at 2, not 8 — the key exception to the “octet”.
• Bonding: the stable, full-shell arrangement that other atoms reach by gaining, losing, or sharing electrons is a noble-gas configuration like helium’s.

Position in the periodic table

• Symbol: He
• Atomic number: 2
• Group 18 (0/VIII), Period 1
• Electron configuration: 2 (a full outer shell with just 2 electrons)
• Monatomic: exists as single atoms

Physical properties

Property	Detail
State at room temperature	Gas
Colour	Colourless
Smell	Odourless
Boiling point	-269 C (lowest of any element)
Density	0.164 g/dm3 (much less dense than air)
Reactivity	Completely unreactive
Toxicity	Non-toxic

Why helium is unreactive

Helium has a full outer shell with just 2 electrons — the stable arrangement for the first shell, which holds a maximum of 2. Like all noble gases, helium has no tendency to gain, lose, or share electrons, so it forms no compounds.

Helium is the only noble gas with 2 outer electrons; every other noble gas has 8.

Uses

Use	Property exploited
Party and weather balloons	Low density (floats), non-flammable
Airships and blimps	Low density, non-flammable (unlike hydrogen)
Deep-sea diving mixtures (with O2)	Low solubility in blood, reducing decompression sickness (“the bends”)
Cooling superconducting magnets (MRI)	Liquid He reaches -269 C
Leak detection	Small atoms slip through tiny gaps

Helium vs hydrogen for balloons

Feature	Helium	Hydrogen
Density	Very low (Mr = 4)	Lowest possible (Mr = 2)
Flammability	Non-flammable	Highly flammable
Safety	Safe	Dangerous (explosive in air)
Cost	More expensive	Cheap to produce
Lifting ability	Slightly less than H2	Maximum

Helium is chosen for safety, even though hydrogen lifts slightly better. The Hindenburg disaster (1937) showed the danger of hydrogen in airships.

Helium in the noble gas trend

Helium has the lowest boiling point of any element (-269 C) because:

1. Helium atoms are the smallest of the noble gases
2. They have the fewest electrons
3. The intermolecular (van der Waals) forces are the weakest
4. Very little energy is needed to separate the atoms

Down Group 0 the boiling points rise as atoms get larger and the forces between them strengthen.

Helium on Earth

Helium is rare in the atmosphere (~0.0005%). It is obtained from natural gas deposits, where it collects from the radioactive decay of heavy elements underground. It is a finite resource: once released, it is too light for Earth’s gravity to hold and escapes to space.

Key facts at a glance

Fact	Detail
Symbol	He
Proton number	2
Group / Period	0 (VIII) / 1
Electron configuration	2 (full outer shell)
Bonding	Monatomic; forms no compounds
Key property	Lowest boiling point of any element (-269 C)

Common exam mistakes

• Saying helium obeys the octet / has 8 outer electrons. Helium is the exception: its outer (first) shell is full at 2 electrons.
• Writing helium as He2. Noble gases are monatomic — single atoms, not molecules.
• Balloon reasoning “helium is lighter than hydrogen”. Hydrogen is actually less dense. Helium is preferred because it is non-flammable and therefore safe.
• Placing helium in Group 2. Despite having 2 outer electrons, helium is in Group 0 because that shell is full — it is inert, not a reactive Group 2 metal.

Exam-style questions

Helium is placed in Group 0 of the Periodic Table even though it has only two outer-shell electrons. Explain why this is the correct group for helium. (2 marks)

Mark scheme

• Group 0 elements are unreactive because they have a full outer electron shell [1]
• helium’s outer shell is full with 2 electrons, so it is stable/unreactive like the other noble gases [1]

Examiner note: the trap is to argue “2 outer electrons means Group 2” — the deciding factor is a full shell, not the number of electrons.

Explain why the boiling point of helium is lower than the boiling point of argon. (3 marks)

Mark scheme

• helium atoms are smaller and have fewer electrons than argon atoms [1]
• so the intermolecular (van der Waals) forces between helium atoms are weaker [1]
• less energy is needed to overcome them, giving helium the lower boiling point [1]

Examiner note: the marks are about forces between atoms — there are no bonds to break inside a monatomic gas.