Hydrogen (H) — Atomic Number 1

“What is Hydrogen?” — Hydrogen is a chemical element; it has the symbol H and atomic number 1. It is the lightest element and the most abundant chemical substance in the universe, constituting roughly 75% of all normal (baryonic) matter by mass. On Earth, it is found primarily in compounds such as water (H₂O) and hydrocarbons like methane (CH₄) and petroleum. In its elemental form at room temperature and pressure, hydrogen is a colourless, odourless, tasteless, and highly flammable diatomic gas with the molecular formula H₂.

At a Glance

Etymology & Discovery

The name hydrogen comes from the Greek words hydro (ὕδωρ, meaning “water”) and genes (γενής, meaning “creator” or “former”) — literally “water-former” — because its combustion produces water. The term was coined in 1783 by the French chemist Antoine Lavoisier, who is also credited with formally recognising hydrogen as an element.

However, hydrogen gas was first artificially produced in the early 16th century by the Swiss alchemist Paracelsus (1493–1541), who observed that adding iron to sulfuric acid produced a flammable gas. In 1671, Robert Boyle rediscovered this reaction and described the properties of the resulting “inflammable solution of Mars” (iron in acid). The English scientist Henry Cavendish (1731–1810) is generally credited with the discovery of hydrogen as a discrete substance in 1766. Cavendish collected the gas over mercury, measured its density, and showed that it burned to form water — though he mistakenly believed the gas was “phlogiston” released from the metal rather than a distinct element. Lavoisier later corrected this by demonstrating that water was a compound of hydrogen and oxygen, not an element itself.

Isotopes — The “Shape” of Hydrogen

Hydrogen is unique among the elements in that its three naturally occurring isotopes each have distinct names, and the differences between them are proportionally enormous (a neutron doubles or triples the nuclear mass). No other element’s isotopes differ so dramatically in their physical and chemical behaviour.

Protium (¹H — “Light Hydrogen”)

Protium is the most common isotope. Its nucleus is simply a single proton. Because it lacks neutrons, the protium atom is uniquely small and light. The absence of neutrons also means that protium has a very high magnetic moment, making it the primary nucleus studied in NMR spectroscopy and the basis of MRI (Magnetic Resonance Imaging) in medicine.

Deuterium (²H or D — “Heavy Hydrogen”)

Deuterium was discovered in 1931 by Harold Urey, who earned the 1934 Nobel Prize in Chemistry for this work. Deuterium forms heavy water (D₂O), which has subtly different physical properties — it freezes at 3.8 °C, boils at 101.4 °C, and has a density ~11% greater than normal water. Heavy water is used as a neutron moderator in certain types of nuclear reactors (CANDU reactors). The larger mass of deuterium causes a kinetic isotope effect — chemical reactions involving deuterium proceed more slowly than those involving protium, a phenomenon exploited in studying reaction mechanisms.

Deuterium is not radioactive and poses no radiation hazard. It is separated from normal hydrogen by fractional distillation, electrolysis, or the Girdler sulfide process.

Tritium (³H or T)

Tritium is produced naturally in the atmosphere when cosmic rays interact with nitrogen. It is also produced artificially in nuclear reactors by neutron bombardment of lithium-6: ⁶Li + n → ⁴He + ³H. Because of its relatively short half-life, essentially all primordial tritium (from Earth’s formation) has long since decayed; all natural tritium today is continuously generated by cosmic radiation.

Tritium is used in:

• Self-powered lighting — tritium gas in phosphor-lined tubes (exit signs, watch dials)
• Tritium dating — determining the age of water samples (oceanography, hydrology)
• Fusion energy research — deuterium-tritium (D-T) fusion is the most accessible fusion reaction

The beta radiation from tritium is very low-energy (maximum 18.6 keV) — it cannot penetrate human skin and is stopped by a few centimetres of air. The primary health hazard is inhalation or ingestion.

Physical Properties

Hydrogen is the lightest gas known. At standard temperature and pressure, it has a density of about 0.09 g/L, which is roughly 1/14 the density of air. Because of this, hydrogen-filled balloons rise rapidly — though helium (which is non-flammable) is now preferred for safety.

In its solid state (achieved only at temperatures below −259 °C), hydrogen forms a hexagonal close-packed crystal lattice.

Hydrogen has the highest thermal conductivity of any gas (0.1805 W/(m·K) at 300 K) and diffuses rapidly through many materials.

Chemical Properties & Reactions

Hydrogen burns in air or oxygen with a very pale blue, almost invisible flame at concentrations between 4% and 75% by volume. The reaction is highly exothermic:

$2{\text{H}}_2\text{(g)}+{\text{O}}_2\text{(g)}\to 2{\text{H}}_2\text{O(g)}\Delta H=-572\text{ kJ/mol}$

Because it releases roughly three times the energy per unit mass of petrol, hydrogen is an attractive fuel. The only combustion product is water — making it a zero-carbon fuel at the point of use.

Key Reactions

1. Combustion (the squeaky pop test): $2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$ When a lighted splint is applied to a test tube of hydrogen, the gas burns rapidly with a characteristic “squeaky pop” sound. This is the standard qualitative test for hydrogen.

2. Reaction with Chlorine: ${\text{H}}_2+{\text{Cl}}_2\to 2\text{HCl}$ This reaction is photochemically initiated — it proceeds explosively in bright sunlight or UV light. The product is hydrogen chloride gas, which dissolves in water to form hydrochloric acid.

3. Haber Process (ammonia synthesis): ${\text{N}}_2+3{\text{H}}_2\rightleftharpoons 2{\text{NH}}_3$ Conditions: iron catalyst, 450 °C, 200 atmospheres pressure. This is the most important industrial use of hydrogen, producing ammonia for fertilisers. The Haber-Bosch process is estimated to sustain roughly half the world’s population through the food production it enables.

4. Hydrogenation of alkenes: $\text{RCH=CHR}+{\text{H}}_2\stackrel{\text{Ni, 150°C}}{\to }{\text{RCH}}_2\text{—}{\text{CH}}_2\text{R}$ Hydrogen adds across carbon-carbon double bonds in the presence of a nickel, palladium, or platinum catalyst. This process is used to convert liquid vegetable oils into solid margarine.

5. Reduction of metal oxides: $\text{CuO}+{\text{H}}_2\to \text{Cu}+{\text{H}}_2\text{O}$ Hydrogen acts as a reducing agent at elevated temperatures, reducing oxides of less reactive metals (copper, lead, iron) to the free metal.

6. Reaction with alkali metals (hydride formation): $2\text{Na}+{\text{H}}_2\to 2\text{NaH}$ At elevated temperatures, hydrogen reacts with alkali and alkaline earth metals to form ionic hydrides, in which hydrogen exists as the hydride ion H⁻.

Laboratory Preparation

Hydrogen is commonly prepared in the laboratory by the reaction of a dilute strong acid (typically hydrochloric or sulfuric) with a reactive metal such as zinc:

$\text{Zn(s)}+2\text{HCl(aq)}\to {\text{ZnCl}}_2\text{(aq)}+{\text{H}}_2\text{(g)}$

The gas is collected by downward displacement of water (it is only very slightly soluble) or by upward delivery (it is less dense than air). Alternatively, hydrogen can be produced by the electrolysis of water acidified with a small amount of sulfuric acid:

$2{\text{H}}_2\text{O(l)}\stackrel{\text{electrolysis}}{\to }2{\text{H}}_2\text{(g)}+{\text{O}}_2\text{(g)}$

Industrial Production

Steam Reforming (the dominant method): Most industrial hydrogen is produced by steam reforming of natural gas (methane):

${\text{CH}}_4+{\text{H}}_2\text{O}\stackrel{\text{Ni, 700–1100°C}}{\to }\text{CO}+3{\text{H}}_2$

The carbon monoxide is then further reacted with steam in the water-gas shift reaction:

$\text{CO}+{\text{H}}_2\text{O}\to {\text{CO}}_2+{\text{H}}_2$

The CO₂ can be captured (carbon capture and storage, CCS) to produce “blue hydrogen.” When the CO₂ is not captured, the product is “grey hydrogen.”

Electrolysis of Water (Green Hydrogen): $2{\text{H}}_2\text{O}\to 2{\text{H}}_2+{\text{O}}_2$ When powered by renewable electricity (solar, wind), this produces “green hydrogen” — a zero-carbon fuel. Currently, electrolysis accounts for only about 4% of global hydrogen production due to cost, but it is growing rapidly.

Industrial & Commercial Uses

Biological Role

Hydrogen is a constituent of virtually all biological molecules — water, proteins, carbohydrates, lipids, and nucleic acids all contain hydrogen atoms. The hydrogen bond (an intermolecular attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom) is fundamental to the structure of DNA (base pairing), protein folding (secondary structure), and the unique properties of water (high surface tension, high specific heat capacity, ice floating).

Certain microorganisms produce hydrogen gas as a metabolic by-product, which is of research interest for biological hydrogen production as a renewable energy source.

Safety

Hydrogen is extremely flammable and forms explosive mixtures with air over a very wide range of concentrations (4%–75% by volume). It ignites with very low energy — a static electricity spark is sufficient. When mixed with oxygen, the mixture is particularly dangerous (detonating mixture). Because hydrogen flames are nearly invisible in daylight, special thermal imaging cameras are used to detect hydrogen fires.

The Hindenburg disaster of 1937, in which a hydrogen-filled airship caught fire, effectively ended the era of passenger airships.

Liquid hydrogen is a cryogenic liquid and causes severe frostbite on contact with skin.

IGCSE Essentials

1. Test for hydrogen: Lighted splint → “squeaky pop” — the single most commonly tested fact
2. Hydrogen is diatomic (H₂) — always write it as H₂ in equations, not H
3. It is less dense than air — collected by upward delivery or over water
4. Not an alkali metal despite being in Group 1 — it is a non-metal with unique chemistry
5. Burning hydrogen produces only water — $2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$
6. Reducing agent — can reduce metal oxides to metals at elevated temperatures

Sources

• Emsley, J. (2011). Nature’s Building Blocks: An A–Z Guide to the Elements. Oxford University Press.
• Greenwood, N. N. & Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann.
• Hydrogen — Encyclopædia Britannica
• Hydrogen — Wikipedia
• Hydrogen — Ptable.com
• Hydrogen — Royal Society of Chemistry Periodic Table
• Weast, R. C. (ed.) (1984). CRC Handbook of Chemistry and Physics (65th ed.). CRC Press.