Hydrogen
Atomic Data
| Atomic Number | 1 |
| Symbol | H |
| Atomic Weight | 1.008 u |
| Density (STP) | 0.0899 g/L |
| Melting Point | −259.14 °C (14.01 K) |
| Boiling Point | −252.87 °C (20.28 K) |
| Electronegativity | 2.2 (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 stable isotopes. The most abundant is ¹H, comprising 99.9885% of all naturally occurring Hydrogen.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Hydrogen-1 | ¹H | 1 | 0 | 99.9885 | Stable |
| Hydrogen-2 | ²H | 1 | 1 | 0.0115 | Stable |
| Hydrogen-3 | ³H | 1 | 2 | trace | Stable |
Abundance & Occurrence
Hydrogen is present in Earth's crust at approximately 1400 ppm by mass and at approximately 750000 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Flammability: Hydrogen is extremely flammable, with a broad ignition range of 4–75% in air. It burns with a nearly invisible pale-blue flame, making fires difficult to detect visually.
- Explosion risk: Hydrogen-air mixtures can detonate if ignited in confined spaces. The detonability range (18–59% in air) is unusually wide; adequate ventilation and gas-detection systems are mandatory in any enclosed workspace.
- Asphyxiation: As a colourless, odourless gas, hydrogen displaces oxygen without warning. High concentrations in poorly ventilated areas can cause rapid oxygen deficiency, dizziness, and loss of consciousness.
- Cryogenic hazards: Liquid hydrogen is stored at −253°C. Contact with skin or eyes causes severe cryogenic burns. Cryogenic equipment requires insulated PPE and strict no-contact protocols.
- Storage and handling: Compressed hydrogen cylinders must be secured upright, kept away from heat and oxidisers, and fitted with pressure-relief devices. Leak detection should use a combustible gas monitor, not an open flame.
Hydrogen in the Real World
Real-World Uses
- Fuel cells — Hydrogen reacts with oxygen in a fuel cell to generate electricity and heat, emitting only water vapour as a by-product, making it a key technology for zero-emission vehicles, buses, and stationary power generation.
- Ammonia production — The Haber-Bosch process combines hydrogen with atmospheric nitrogen to produce ammonia (NH₃), which is the basis of synthetic fertilisers that sustain food production for billions of people worldwide.
- Petroleum refining — Hydrocracking and hydrodesulfurisation use hydrogen to break down heavy crude oil fractions into lighter fuels and to remove sulfur compounds, improving fuel quality and reducing emissions from combustion.
- Rocket propellant — Liquid hydrogen serves as a high-energy fuel in cryogenic rocket engines, including those used on NASA's Space Launch System. Its very low molecular weight gives it exceptional specific impulse compared to hydrocarbon fuels.
- Hydrogen peroxide production — Industrial hydrogen peroxide (H₂O₂) is manufactured via the anthraquinone oxidation process, which requires hydrogen. It is widely used as a bleaching agent, disinfectant, and chemical oxidiser.
- Metal processing — Hydrogen atmospheres are used in the heat treatment of metals to prevent oxidation and to reduce metal oxides, producing high-purity metals for electronics and speciality alloys.
- Food industry — Catalytic hydrogenation of vegetable oils converts liquid unsaturated fats into solid or semi-solid fats used in margarine and cooking shortenings.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is hydrogen used for?
Hydrogen has many industrial and emerging energy uses. It is the primary feedstock for ammonia production via the Haber-Bosch process, a key reagent in petroleum refining, and a propellant in rocket engines. Increasingly, hydrogen is used in fuel cells to generate electricity for vehicles and stationary power systems, producing only water as a by-product.
Is hydrogen flammable?
Yes, hydrogen is highly flammable. It burns with a nearly invisible pale-blue flame in air and has a wide flammability range of 4–75% concentration in air, which is broader than most common fuels. Its low ignition energy means even a small spark can ignite a hydrogen-air mixture, so appropriate ventilation and leak detection are essential when handling it.
How many isotopes does hydrogen have?
Hydrogen has three naturally occurring isotopes. Protium (¹H) has no neutrons and accounts for 99.9885% of natural hydrogen. Deuterium (²H) has one neutron and makes up 0.0115%. Tritium (³H) has two neutrons, is radioactive with a half-life of 12.32 years, and occurs only in trace amounts produced by cosmic-ray interactions in the upper atmosphere.
Who discovered hydrogen?
Hydrogen was first isolated and recognised as a distinct element by Henry Cavendish in 1766. He produced hydrogen gas by reacting metals with acids and measured its properties, noting it was far lighter than air. Antoine Lavoisier later named the element 'hydrogen' in 1783, from the Greek words for 'water-forming', after demonstrating that burning hydrogen produces water.
Why is hydrogen the most abundant element in the universe?
Hydrogen is the simplest element, consisting of just one proton and one electron, and it was the first element formed after the Big Bang during the process known as Big Bang nucleosynthesis. Because it requires the least energy to form and no heavier elements were yet present, hydrogen became overwhelmingly dominant, making up approximately 75% of all normal matter in the universe by mass.