1 H Hydrogen 1.008
Nonmetal s-block Period 1 Group 1

Hydrogen

H · Element 1 · Alkali metals

The simplest atom there is — one proton, one electron — and about 90% of every atom in the universe.

STATE AT 20°C Gas
ATOMIC MASS 1.008 u
ELECTRON CONFIGURATION 1s¹
All 118 2He

Structure

The hydrogen atom

Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.

Orbital cloud

Measured values

Property sheet

Every bar shows where hydrogen sits among all 118 elements for that property.

Physical

Density 0 g/cm³ 0%
Melting point 14 K 1%
Boiling point 20.3 K 1%
Specific heat 14.304 J/g·K
Thermal conductivity 0.18 W/m·K 13%

Atomic

Atomic radius 53 pm 3%
Covalent radius 31 pm
Van der Waals radius 120 pm

Electronic

Electronegativity 2.2 78%
Ionisation energy 1312.2 kJ/mol 94%
Electron affinity 72.4 kJ/mol 67%

Occurrence

Abundance in crust 1400 mg/kg 91%

Identity

SymbolH
Atomic number1
Atomic mass1.008 u
CategoryNonmetal
Blocks
Crystal structurehexagonal
Oxidation states-1, +1
Discovered1766
Discovered byHenry Cavendish

Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.

Size, to scale

How big is a hydrogen atom?

Radius 53 pm — that is 0.053 nm, so about 9434 million of them side by side would span a millimetre.

Thermal range

Solid, liquid, gas — and when

Hydrogen is liquid over a 6 K window, from 14 K to 20 K.

Where it sits

Position in the table

Hydrogen sits in period 1, group 1. Everything in group 1 shares the same outer-electron count, which is why they behave so similarly.

OTHER NONMETALS

6C 7N 8O 15P 16S 34Se
All nonmetals

The story

What hydrogen is, and how we found it

The Universe's Foundation Element

Hydrogen stands as the simplest and most abundant element in the universe, comprising approximately 75% of all normal matter. With just one proton and one electron, it serves as the fundamental building block from which all other elements are forged in the hearts of stars.

Physical Properties

At standard conditions, hydrogen exists as a colorless, odorless, tasteless diatomic gas (H₂). It's the lightest element with a density 14 times less than air, causing hydrogen-filled balloons to rise rapidly. The element becomes liquid at an extremely cold -253°C (-423°F) and solid at -259°C (-434°F).

Chemical Characteristics

Hydrogen exhibits unique chemical behavior, acting as both a reducing agent and oxidizing agent depending on reaction conditions. It readily forms covalent bonds with nonmetals and ionic compounds with active metals. The H-H bond energy of 436 kJ/mol makes H₂ relatively stable, requiring activation energy for most reactions.

Isotopic Varieties

Protium (¹H): 99.985% abundance - the "normal" hydrogen with no neutrons

Deuterium (²H or D): 0.015% abundance - "heavy hydrogen" with one neutron, used in nuclear fusion

Tritium (³H or T): Trace amounts - radioactive with two neutrons, half-life of 12.3 years

Applications

What hydrogen is used for

Significant Industrial Applications

🚀 Space & Energy Technologies

  • Rocket Fuel: Liquid hydrogen powers NASA's Space Launch System and SpaceX Starship upper stages, providing the highest specific impulse (efficiency) of any chemical fuel
  • Fuel Cells: Toyota Mirai and Hyundai NEXO vehicles use hydrogen fuel cells, converting H₂ + O₂ → H₂O + electricity with zero emissions
  • Green Energy Storage: Excess renewable energy electrolyzes water to produce hydrogen for later power generation

🏭 Chemical Manufacturing

  • Ammonia Production (Haber Process): N₂ + 3H₂ → 2NH₃ at 450°C and 200 atmospheres, producing 175 million tons annually for fertilizers
  • Methanol Synthesis: CO + 2H₂ → CH₃OH using copper-zinc catalysts at 250°C, creating plastics and fuels
  • Hydrogenation: Converting unsaturated fats to saturated fats in margarine and shortening production using nickel catalysts
  • Hydrocracking: Breaking down heavy petroleum molecules into gasoline and diesel using hydrogen at 400°C and 100+ atmospheres

🔬 Advanced Technologies

  • Semiconductor Manufacturing: Ultra-pure hydrogen creates reducing atmospheres for silicon chip production
  • Float Glass Process: Hydrogen-nitrogen atmosphere prevents oxidation during window glass manufacturing
  • Metal Processing: Hydrogen reduces metal oxides to pure metals: CuO + H₂ → Cu + H₂O
  • Welding: Atomic hydrogen welding reaches 4000°C by recombining H atoms at the weld point

Handling

Safety

Comprehensive Safety Information

⚠️ Primary Hazards

FLAMMABLE GAS - EXTREME FIRE HAZARD

  • Ignition Range: 4-75% in air (widest range of any gas)
  • Autoignition Temperature: 500°C (932°F)
  • Flame Speed: Up to 3.46 m/s - extremely fast burning
  • Invisible Flame: Hydrogen burns with a nearly invisible pale blue flame in daylight

🛡️ OSHA Exposure Limits

  • No established exposure limits - hydrogen is physiologically inert
  • Asphyxiation Risk: Concentrations >25% can displace oxygen
  • Simple Asphyxiant: Dangerous in confined spaces by oxygen displacement

🧤 Personal Protective Equipment

  • Eye Protection: Safety glasses or face shield for liquid hydrogen
  • Skin Protection: Thermal-insulated gloves for cryogenic liquid hydrogen
  • Respiratory Protection: Self-contained breathing apparatus in >10% concentrations
  • Clothing: Loose-fitting to prevent static buildup; no synthetic materials

📦 Storage Requirements

  • Gas Storage: Steel cylinders at 150-200 bar pressure
  • Liquid Storage: Double-walled vacuum-insulated dewars at -253°C
  • Location: Well-ventilated areas, away from ignition sources
  • Grounding: All equipment must be grounded to prevent static discharge
  • Detection: Hydrogen sensors required (catalytic bead or thermal conductivity types)

🚨 Emergency Procedures

Gas Leak:

  • Evacuate area immediately - hydrogen rises and accumulates at ceiling level
  • Eliminate all ignition sources within 50 feet
  • Ventilate area from top down
  • Use non-sparking tools only

Fire:

  • DO NOT extinguish hydrogen flames unless source can be stopped
  • Cool surrounding equipment with water spray
  • Withdraw immediately if container sounds change or discolor

Cryogenic Exposure:

  • Frostbite treatment: warm affected area gradually with lukewarm water
  • Do not rub frostbitten areas
  • Seek immediate medical attention for severe exposure

Quick answers

Hydrogen: common questions

What is Hydrogen?

Hydrogen (symbol H) is element 1 on the periodic table, a nonmetal in period 1, group 1. The simplest atom there is — one proton, one electron — and about 90% of every atom in the universe. At room temperature it is a gas.

What is the electron configuration of Hydrogen?

Hydrogen's ground-state electron configuration is 1s¹, giving 1 occupied shell holding 1 electrons respectively.

What are the melting and boiling points of Hydrogen?

Hydrogen melts at 14 K (-259.1 °C) and boils at 20.3 K (-252.9 °C).

What is the atomic mass of Hydrogen?

The standard atomic weight of Hydrogen is 1.008 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.

How dense is Hydrogen?

Hydrogen is a gas at room temperature; its density is about 0.1 g/L at 0 °C and 1 atm — roughly lighter than air.

What is the electronegativity of Hydrogen?

Hydrogen has a Pauling electronegativity of 2.2. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.

Who discovered Hydrogen, and when?

Hydrogen was discovered in 1766 by Henry Cavendish. It is named after greek hydro + genes, "water-former".

How common is Hydrogen on Earth?

Hydrogen makes up about 0.14% of the Earth's crust — common enough to be mined at scale.

Next → He Helium Element 2 · Noble gas