Atomic number 1

Hydrogen

H
+ phase
− phase

Shells Orbitals 1s

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1 electron in 1 shell: 1.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Nonmetal
Period: 1
Group: 1
Block: s
Standard atomic weight: 1.0080(2) Da
Electron configuration: 1s¹
Phase at 25 °C: gas
Melting point: −259.16 °C
Boiling point: −252.87 °C
Density: 0.08988 g/L
Electronegativity (Pauling): 2.2
Discovered: 1766, Henry Cavendish

Neighbours in the table

A hydrogen atom has one electron, and the atom model reads out exactly that: one electron in one shell. The electron sits in the 1s orbital, the lowest-energy region an electron can occupy around a nucleus. A full first shell holds two electrons, so hydrogen’s is half filled and half empty at once. Take the electron away and a bare proton is left, H⁺, which is why the table prints hydrogen in group 1. Add one instead and the shell is full, giving the hydride ion H⁻, which is how a group 17 element behaves.

PubChem lists +1 and −1 as hydrogen’s common oxidation states, a bookkeeping charge that assumes fully ionic bonds. OpenStax calls hydrogen a unique element that almost deserves a place of its own in the table. Pulling the electron off a hydrogen atom takes 13.598 eV, the ionisation energy on PubChem’s element record. The free element is dihydrogen, two hydrogen atoms joined by a single bond.

The name comes from Greek: hydro, water, and genes, forming, because the gas burns in air to form water. The gas itself was known long before that. Robert Boyle produced it from iron and acids as early as 1671, and Henry Cavendish recognised hydrogen as a distinct element in 1766.

Where it is found

About nine-tenths of the visible universe is hydrogen, by Jefferson Lab’s estimate, and Los Alamos puts hydrogen’s share of the mass at three quarters. A star is mostly that hydrogen fusing into helium, and the sun has roughly five billion years of its own left to burn. Jupiter is mostly hydrogen too, and deep inside the planet, Los Alamos says, the pressure turns molecular hydrogen metallic.

On Earth the element is found in compounds, and water is the most abundant of them. A kilogram of crust averages 1.40×10³ milligrams on PubChem’s record, about 0.14 % by mass, and a litre of seawater 1.08×10⁵ milligrams. Nearly every one of those atoms is protium, the lightest of hydrogen’s three isotopes, which differ only in the neutrons in the nucleus. NIST measures protium at 0.999885 of the total, deuterium at 0.000115.

That mixture is not the same everywhere. The isotopic composition of rain, rivers and tap water shifts with elevation, season and distance from the coast. That is why the standard atomic weight above, 1.008 Da, is a conventional value.

Why it matters

The world used 97 million tonnes of hydrogen in 2023, the International Energy Agency reports, most of it in oil refining and the chemical industry. In the chemical industry the main product is ammonia, which becomes fertiliser and nitric acid. The Haber process joins nitrogen from the air with hydrogen over a catalyst, releasing 92 kJ for every two moles of ammonia it forms.

Burning hydrogen in oxygen releases 286 kJ for every mole of water formed, and the water is the only product. OpenStax puts that forward as the case for hydrogen as a non-polluting fuel. The same heat of combustion lets an oxygen-hydrogen welding torch reach 2800 °C. Paired with liquid oxygen, liquid hydrogen is a rocket propellant. The liquid exists only between −259.16 °C and −252.87 °C, which puts it to work in cryogenics, the science of temperatures near absolute zero.

Hydrogen belongs to more compounds than any other element: it is in petroleum and in starch, in fats and oils, in alcohols and acids. One of them is methane, table sugar another. Hydrogenation adds hydrogen to fats and oils, peanut oil among them. The hydrogen chloride made from it becomes hydrochloric acid, used to make metal chlorides, dyes, glue and glucose.

Properties

Properties of hydrogen Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	1
Standard atomic weight	1.0080(2) Da	Da	IUPAC/CIAAW 2024
Melting point	−259.16 °C	°C	Los Alamos: −259.16 °C
Boiling point	−252.87 °C	°C	PubChem: 20.28 K
Density	0.08988 g/L	g/L
Electronegativity (Pauling)	2.2
Electron configuration	1s¹
Phase at 25 °C	gas

The standard atomic weight of this element is an interval in nature, from 1.00784 to 1.00811 Da; the value shown is IUPAC's conventional abridged value.

Safety

Hydrogen’s hazard is fire, not poisoning. PubChem’s safety summary gives it the signal word Danger and the statement H220, extremely flammable gas; NFPA rates it 4 for fire against 0 for health. Notifications from industry add H280, a compressed gas that may explode if heated, and H281, refrigerated hydrogen that causes cryogenic burns. Those hazards fall on refineries, ammonia plants and the insulated containers that carry the cold liquid; PubChem’s Laboratory Chemical Safety Summary for hydrogen, CID 783, sets out those hazards.

Questions

How is hydrogen made?

Hydrogen is split out of compounds rather than mined, and on Earth water holds more of it than any other compound. Passing steam over coke at 1000 °C gives water gas, a mixture of carbon monoxide and hydrogen. Steam reforming is one of the main commercial processes: hydrocarbons from natural gas or petroleum pass with steam over a nickel-based catalyst. Direct current through acidified water splits the water and releases hydrogen as well.

What is deuterium, and how common is it?

Deuterium is hydrogen's stable heavy isotope, found in 1932, and its nucleus holds a neutron beside the proton. NIST measures natural hydrogen as 0.999885 protium to 0.000115 deuterium. OpenStax gives one deuterium atom for every 7000 hydrogen atoms, and Los Alamos about one in 6000. Collected as heavy water, deuterium is the moderator and coolant in CANDU nuclear reactors.

Why do hydrogen balloons float?

One litre of the gas weighs 0.08988 g, far less than the same volume of air, so the air around a balloon pushes it up. Los Alamos puts the lifting power of one cubic foot at roughly 0.07 lb and still lists balloon filling among hydrogen's uses. Its GHS record also carries the signal word Danger and the statement H220, extremely flammable gas.

Is hydrogen gas dangerous?

The hazard is fire, not poisoning. PubChem's safety summary states that hydrogen is not toxic but acts as a simple asphyxiant, displacing the oxygen in the air of a closed space. It burns across a wide span of mixtures with air, from 4 to 75 per cent by volume, and the flame it burns with is pale blue and almost invisible.

Compounds containing hydrogen

HBr Hydrogen bromide 2 atoms, 1 single bond.
HCl Hydrogen chloride 2 atoms, 1 single bond.
HF Hydrogen fluoride 2 atoms, 1 single bond.
H₂S Hydrogen sulfide 3 atoms, 2 single bonds.
KOH Potassium hydroxide 3 atoms in one formula unit of an ionic lattice.
NaOH Sodium hydroxide 3 atoms in one formula unit of an ionic lattice.
H₂O Water 3 atoms, 2 single bonds, bent.
C₂H₂ Acetylene 4 atoms, 3 bonds.

All 134 compounds containing hydrogen

NH₃ Ammonia 4 atoms, 3 single bonds, trigonal pyramidal.
CH₂O Formaldehyde 4 atoms, 3 bonds.
H₂O₂ Hydrogen peroxide 4 atoms, 3 single bonds.
HNO₂ Nitrous acid 4 atoms, 3 bonds.
Ca(OH)₂ Calcium hydroxide 5 atoms in one formula unit of an ionic lattice.
CHCl₃ Chloroform 5 atoms, 4 single bonds, tetrahedral.
CH₂Cl₂ Dichloromethane 5 atoms, 4 single bonds, tetrahedral.
HCOOH Formic acid 5 atoms, 4 bonds.
Mg(OH)₂ Magnesium hydroxide 5 atoms in one formula unit of an ionic lattice.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
HNO₃ Nitric acid 5 atoms, 4 bonds.
NH₄Cl Ammonium chloride 6 atoms in one formula unit of an ionic lattice.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
NaHCO₃ Sodium bicarbonate 6 atoms in one formula unit of an ionic lattice.
C₂H₃Cl Vinyl chloride 6 atoms, 5 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
H₂SO₄ Sulfuric acid 7 atoms, 6 bonds, tetrahedral.
C₂H₂F₄ 1,1,1,2-Tetrafluoroethane 8 atoms, 7 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
H₃PO₄ Phosphoric acid 8 atoms, 7 bonds.
CH₃COONa Sodium acetate 8 atoms in one formula unit of an ionic lattice.
CO(NH₂)₂ Urea 8 atoms, 7 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₂H₅NO₂ Glycine 10 atoms, 9 bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₄N₂O₂ Uracil 12 atoms, 12 bonds.
C₃H₇NO₂ Alanine 13 atoms, 12 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₄H₅N₃O Cytosine 13 atoms, 13 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
(CH₃)₃N Trimethylamine 13 atoms, 12 single bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
C₃H₇NO₂S Cysteine 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₅H₅N₅ Adenine 15 atoms, 16 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₃H₆N₆ Melamine 15 atoms, 15 bonds.
C₅H₆N₂O₂ Thymine 15 atoms, 15 bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₅H₅N₅O Guanine 16 atoms, 17 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₅H₄N₄O₃ Uric acid 16 atoms, 17 bonds.
C₅H₉N₃ Histamine 17 atoms, 17 bonds.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₃H₈NO₅P Glyphosate 18 atoms, 17 bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
C₅H₉NO₄ Glutamic acid 19 atoms, 18 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₅H₈NO₄Na Monosodium glutamate 19 atoms in one formula unit of an ionic lattice.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₈H₉NO₂ Acetaminophen 20 atoms, 20 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₄H₁₁N₅ Metformin 20 atoms, 19 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₈H₁₁NO₂ Dopamine 22 atoms, 22 bonds.
C₆H₁₃NO₂ Isoleucine 22 atoms, 21 bonds.
C₆H₁₃NO₂ Leucine 22 atoms, 21 bonds.
C₉H₁₁NO₂ Phenylalanine 23 atoms, 23 bonds.
C₈H₁₀N₄O₂ Caffeine 24 atoms, 25 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₉H₁₁NO₃ Tyrosine 24 atoms, 24 bonds.
C₁₀H₁₂N₂O Serotonin 25 atoms, 26 bonds.
C₉H₁₃NO₃ Epinephrine 26 atoms, 26 bonds.
C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₁₀H₁₄N₂ Nicotine 26 atoms, 27 bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.
C₁₁H₁₂N₂O₂ Tryptophan 27 atoms, 28 bonds.
C₁₂H₁₇NO DEET 31 atoms, 31 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₃H₁₆N₂O₂ Melatonin 33 atoms, 34 bonds.
C₁₅H₁₁I₄NO₄ Thyroxine 35 atoms, 36 bonds.
C₁₃H₂₁NO₃ Albuterol 38 atoms, 38 bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₄H₁₈N₂O₅ Aspartame 39 atoms, 39 bonds.
C₁₄H₂₂N₂O Lidocaine 39 atoms, 39 bonds.
C₁₇H₂₁NO Diphenhydramine 40 atoms, 41 bonds.
C₁₇H₁₈F₃NO Fluoxetine 40 atoms, 41 bonds.
C₁₆H₁₈N₂O₄S Penicillin G 41 atoms, 43 bonds.
C₁₆H₁₉N₃O₅S Amoxicillin 44 atoms, 46 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₀H₁₆N₅O₁₃P₃ Adenosine triphosphate 47 atoms, 49 bonds.
C₂₀H₂₄N₂O₂ Quinine 48 atoms, 51 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.
C₃₃H₃₅FN₂O₅ Atorvastatin 76 atoms, 79 bonds.

Appears in these articles

An acid gives a proton and a base takes it: Arrhenius, Brønsted–Lowry and Lewis Hydrogen chloride gives its proton to water effectively completely; acetic acid gives only a small fraction.
What holds atoms together: ionic, covalent and metallic bonds Table salt has no molecules in it; hydrogen chloride gas is nothing but molecules.