Helium (He) — Atomic Number 2
Helium is a chemical element with the symbol He and atomic number 2. It is the first noble gas and the second-lightest element. It is a colourless, odourless, chemically inert monatomic gas. After hydrogen, it is the second most abundant element in the universe, but on Earth it is relatively rare — found primarily in natural gas deposits.
At a Glance
| Property | Value |
|---|---|
| Symbol | He |
| Atomic Number (Z) | 2 |
| Mass Number (A) | 4 |
| Protons | 2 |
| Neutrons | 2 |
| Electrons | 2 |
| Electron Configuration | 2 |
| Group | 18 / 0 (Noble Gas) |
| Period | 1 |
| State at RTP | Colourless, odourless gas |
| Boiling Point | −269 °C (lowest of any element) |
| Density | 0.18 g/L (much less dense than air) |
Etymology & Discovery
The name comes from Greek helios meaning “sun” — helium was first detected in the spectrum of the Sun’s chromosphere during a solar eclipse in 1868 by French astronomer Pierre Janssen and English astronomer Norman Lockyer. It was not found on Earth until 1895, when Scottish chemist William Ramsay isolated it from a uranium mineral.
Why Helium is Unreactive
Helium has a full outer shell of 2 electrons. It has no tendency to gain, lose, or share electrons. All noble gases share this property — they are the least reactive group on the periodic table.
Chemical Properties
Helium has no chemical reactions under normal conditions. It does not burn, does not react with acids or bases, and forms no stable compounds. It exists as single atoms (monatomic: He, never He₂).
Key Uses
- Balloons and airships — safe alternative to flammable hydrogen
- MRI scanners — liquid helium cools the superconducting magnets
- Welding — inert shielding gas for arc welding
- Deep-sea diving — helium-oxygen mixtures (heliox) prevent nitrogen narcosis
IGCSE Essentials
- Monatomic — He, never He₂
- Full outer shell of 2 electrons explains its unreactivity
- Less dense than air — balloons rise
- Non-flammable — compare with hydrogen on safety questions