Hydrogen
Atomic Data
| Atomic Number | 1 |
| Symbol | H |
| Atomic Weight | 1.008 u |
| Density (STP) | 0.0899 g/L |
| Melting Point | −259.16 °C (13.99 K) |
| Boiling Point | −252.88 °C (20.27 K) |
| Electronegativity | 2.20 (Pauling) |
| Electron Config. | 1s¹ |
| Oxidation States | −1, +1 |
| Phase at STP | Gas |
| Category | Reactive nonmetal |
| Period / Group | 1 / 1 |
| CAS Number | 1333-74-0 |
Electron Configuration
1s1
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 1 | 1 |
| Total | 1 | 1 | ||
Isotopes of Hydrogen
Hydrogen has three naturally occurring isotopes. Two are stable; tritium is radioactive with a 12.32-year half-life.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Protium | ¹H | 1 | 0 | 99.98% | Stable |
| Deuterium | ²H (D) | 1 | 1 | 0.02% | Stable |
| Tritium | ³H (T) | 1 | 2 | Trace | Radioactive (t½ = 12.32 yr) |
Abundance & Occurrence
Hydrogen is the most abundant element in the universe. On Earth it is almost always found bound in compounds rather than as free gas.
Composition of the Universe (By Mass)
Composition of the Human Body (By Mass)
Discovery & History
Safety & Handling
- Not toxic but an asphyxiant — displaces oxygen in enclosed spaces.
- Highly flammable: 4–75% flammability range in air. Colourless and odourless — leaks require sensors to detect.
- Hydrogen embrittlement: prolonged exposure weakens steel and pressure vessels over time.
- Hydrogen sulfide (H₂S) — a common hydrogen compound — is highly toxic with a rotten-egg odour.
- Always handle in well-ventilated areas with hydrogen-rated detection equipment.
Hydrogen in the Real World
Real-World Uses
- Fertiliser production — Combined with nitrogen via the Haber process to produce ammonia, the basis for most synthetic fertilisers.
- Petroleum refining — Used in hydrocracking and hydrodesulfurisation to refine crude oil and remove sulfur contaminants.
- Hydrogen fuel cells — Reacts with oxygen to generate electricity; only byproduct is water.
- Rocket propellant — Liquid hydrogen powers space launch vehicles including NASA's Space Launch System.
- Food industry — Hydrogenation of vegetable oils creates margarine and extends shelf life.
- Electronics manufacturing — Used as a carrier gas and reducing atmosphere in semiconductor fabrication.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is hydrogen used for?
Hydrogen is used primarily for petroleum refining, ammonia production for fertilisers, rocket propellant, hydrogen fuel cells, and the hydrogenation of vegetable oils. It is the most promising clean energy carrier for the future.
Is hydrogen the most abundant element in the universe?
Yes. Hydrogen makes up approximately 75% of all normal matter in the universe by mass, and is the primary component of stars including the Sun.
What is the difference between hydrogen and deuterium?
Deuterium (²H) is a stable isotope of hydrogen containing one proton and one neutron, giving it twice the mass of ordinary hydrogen (protium). It makes up about 0.02% of all natural hydrogen.
Is hydrogen safe to use as a fuel?
Hydrogen can be used safely with proper handling. It is highly flammable (4–75% in air) and leaks are invisible. When used in fuel cells it produces only water as a byproduct, making it very clean.
Who discovered hydrogen?
Henry Cavendish first isolated and characterised hydrogen in 1766. Antoine Lavoisier named it ‘hydrogen’ in 1783, from Greek hydro (water) + genes (forming).