Chemistry 2e · Representative Metals, Metalloids, and Nonmetals
Occurrence, Preparation, and Compounds of Hydrogen
On this page 9 sections
In 30 seconds
Hydrogen is the simplest and most abundant element in the universe: roughly three-quarters of ordinary matter by mass is hydrogen, and it fuels the stars through fusion. On Earth, however, free H₂ gas is rare — hydrogen is too reactive to linger uncombined. It is found everywhere in compounds: water (H₂O), hydrocarbons, biomass, and acids, ranking ninth among crustal elements by mass.
Hydrogen's position atop group 1 is a historical accident of electron configuration, but its chemistry is unlike any alkali metal's. With an electronegativity of 2.20 — between boron and carbon — hydrogen can do all three things an atom can do with electrons: lose one (H⁺, the proton, in acids), gain one (H⁻, the Hydride A compound of hydrogen with another element Full entry → ion, in ionic hydrides like NaH), and share one (in the covalent bonds of H₂, H₂O, NH₃, CH₄). No other element spans this full range.
Three isotopes exist: Protium ¹H — one proton, no neutrons Full entry → (¹H, no neutron — over 99.98% of natural hydrogen), Deuterium ²H (D) — proton + neutron Full entry → (²H or D, one neutron, ~0.0156% abundance), and Tritium ³H (T) — proton + 2 neutrons, radioactive β⁻ emitter Full entry → (³H or T, two neutrons, radioactive with a β⁻ half-life of about 12.3 years). Because deuterium is twice as massive as protium, its compounds (heavy water, D₂O) behave measurably differently — exploited in reactors, NMR, and Isotope Same element, different number of neutrons tracing.
Why this matters
- Energy transition: hydrogen is a leading clean-fuel candidate — burning it produces only water, and fuel cells convert H₂ directly into electricity. But "green hydrogen" is only as clean as the method used to make it.
- Food security: roughly half of the world's dietary nitrogen comes from synthetic fertilizer made by the Haber process, which consumes enormous quantities of H₂.
- Industrial Hydrogenation Adding H₂ across a double bond Full entry →: H₂ adds across C=C double bonds to turn liquid vegetable oils into margarines and shortenings; the same chemistry matters in petrochemical refining.
- Acids and biology: every acid is a hydrogen compound; proton transfer is the essence of acid–base chemistry, and the proton gradient across mitochondrial membranes drives ATP synthesis.
- Exams: expect the three isotopes, hydride types, laboratory vs. industrial preparation, and H₂ stoichiometry.
The college version
Core Concepts
Occurrence and isotopes
Hydrogen is the most abundant element in the universe (~75% by mass), but on Earth it exists almost entirely combined. The isotope mix matters: protium dominates; deuterium (²H, D) is present at roughly 150 ppm; tritium (³H) is produced naturally in trace amounts by cosmic rays and in reactors (used in fusion research and luminous devices). Tritium's β⁻ decay (half-life ≈ 12.3 years) is why heavy-water reactor moderators are monitored for tritium buildup.
Chemical personality: the three faces of hydrogen
- H⁺ (proton): lost to very electronegative elements — acids: HCl → H+ + Cl-. A bare proton never exists alone in water; it forms H₃O⁺ (hydronium).
- H⁻ (hydride): gained from very active metals — ionic hydrides: 2Na + H2 → 2NaH, Ca + H2 → CaH2. H⁻ is a powerful reducing agent and reacts vigorously with water: NaH + H2O → NaOH + H2.
- Shared electron (covalent): with elements of similar electronegativity — H₂, H₂O, NH₃, CH₄, HCl, H₂S. Most hydrogen in the world is in this category.
This behavior maps onto three classes of hydrides: ionic (NaH, CaH₂ — salt-like solids), covalent (CH₄, NH₃, H₂O, HCl — molecular), and metallic/interstitial (TiH₂; palladium absorbs many hundreds of times its own volume of H₂ and is used to purify the gas).
Laboratory preparation
The classic laboratory route is the reaction of an active metal with a dilute acid (a single-displacement reaction):
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
(equivalently, Zn + H2SO4 → ZnSO4 + H2). General principle: metals above hydrogen in the activity series displace H₂ from acids; zinc is preferred in teaching labs because it reacts at a manageable rate. Because H₂ forms explosive mixtures with air, such preparations need proper ventilation and ignition controls, with the gas collected over water and open flames excluded.
Industrial preparation
- Steam reforming Industrial H₂ production from methane + steam Full entry → of methane (the dominant route): CH4(g) + H2O(g) → CO(g) + 3H2(g), at high temperature over a nickel catalyst; the CO is then converted by the water-gas shift, CO(g) + H2O(g) → CO2(g) + H2(g). Together these strip hydrogen from natural gas and water.
- Electrolysis of water Splitting H₂O into H₂ and O₂ with electricity Full entry →: 2H2O(l) → 2H2(g) + O2(g). This yields very pure H₂ and is the basis of "green hydrogen" when powered by renewable electricity, but it currently costs more than steam reforming.
- From metal + steam (minor): hot iron or carbon reacts with steam to give H₂ and an oxide.
Reactions and uses
Hydrogen is a superb reducing agent and fuel:
- Combustion: 2H2(g) + O2(g) → 2H2O(g) — very exothermic; powers rockets and, in reverse (fuel cells), produces electricity with water as the only emission.
- With halogens: H2 + Cl2 → 2HCl; with nitrogen (Haber process) to make ammonia.
- Hydrogenation: H₂ adds across double bonds — vegetable oil + H₂ → margarine; also removes sulfur from petroleum (hydrodesulfurization).
- With metal oxides: CuO + H2 → Cu + H2O — hydrogen reduces metal oxides to metals.
Safety note (general principle): H₂ is colorless, odorless, burns with an almost invisible flame, and forms explosive mixtures with air over a wide composition range — so leak detection, ventilation, and exclusion of ignition sources are essential. No step-by-step procedures are given here; always follow institutional safety protocols.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Hydrogen (element) | Hydride (ion/compound) | Hydrogen is the element H; a hydride is a compound containing H⁻ (ionic) or covalently bonded H. |
| H⁺ (bare proton) | H₃O⁺ (hydronium) | A bare proton never exists alone in water; it attaches to a water molecule to form hydronium. |
| Electrolysis of water | Electrolysis of melts | Water electrolysis gives H₂ + O₂ from water; melt electrolysis (Downs/Hall–Héroult) extracts metals. |
| Deuterium and tritium | Same abundance and behavior | D is stable and ~0.0156% abundant; T is radioactive (β⁻, 12.3 yr half-life) and trace. |
| "Green hydrogen" | All hydrogen production | Only hydrogen from renewable-powered electrolysis (or low-carbon methods) is "green"; most H₂ today comes from fossil methane. |
| H₂ as an energy source | H₂ as a fuel source | H₂ is an energy carrier — it must be produced from something else (water, methane); it is not mined like coal. |
| All metals + acid give H₂ | Only metals above hydrogen | Metals below H in the activity series (Cu, Ag, Au) do not displace H₂ from acids. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Hydrogen is the lightest and most common element in the universe — it's what the Sun burns. It's a bit like a social chameleon: sometimes it gives away its one electron (like in acid), sometimes it takes an extra one (like in a battery), and sometimes it shares (like in water). By itself it's a super-light, super-flammable gas, which is why people dream of using it as clean fuel — burning it makes only water.
Worked example
Example 1: Laboratory hydrogen — how much gas from a given mass of zinc?
Zinc reacts with hydrochloric acid:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Problem: What volume of H₂ (at STP, 0 °C, 1 atm) is produced when 25.0 g of Zn reacts with excess HCl? M(Zn) = 65.38 g mol-1, M(H2) = 2.016 g mol-1, molar volume Vm = 22.4 L mol-1.
Plan: n = mM; 1:1 mole ratio of Zn to H₂; V = n × Vm.
Substitution (dimensional analysis):
n(Zn) = 25.0 g × 1 mol65.38 g = 0.3824 mol
n(H2) = 0.3824 mol Zn × 1 mol H21 mol Zn = 0.3824 mol
V(H2) = 0.3824 mol × 22.4 L1 mol = 8.57 L
Answer: about 8.57 L of hydrogen gas at STP.
Example 2: Industrial hydrogen — what does steam reforming of 100 g of methane yield?
Steam reforming converts methane and steam to hydrogen:
CH4(g) + H2O(g) → CO(g) + 3H2(g)
Problem: How many grams of H₂ can be produced from 100.0 g of CH₄ with excess steam (assuming complete conversion)? M(CH4) = 12.01 + 4(1.008) = 16.04 g mol-1, M(H2) = 2.016 g mol-1.
Plan: n = mM, ratio 3 mol H21 mol CH4, then m = n × M.
Substitution:
n(CH4) = 100.0 g × 1 mol16.04 g = 6.234 mol
n(H2) = 6.234 mol CH4 × 3 mol H21 mol CH4 = 18.70 mol
m(H2) = 18.70 mol × 2.016 g1 mol = 37.7 g
Answer: 37.7 g of H₂ from 100.0 g of methane — about 38% of the methane's mass, with CO also produced (the water-gas shift converts it to more H₂ and CO₂). This is why natural gas, not water, is the main feedstock for hydrogen production.
Key takeaways
- H is the most abundant element in the universe (~75% by mass); on Earth it is combined in water, hydrocarbons, and acids — free H₂ is rare.
- Isotopes: ¹H (protium, >99.98%), ²H (deuterium, ~0.0156%), ³H (tritium, radioactive, β⁻, half-life ≈ 12.3 yr).
- Hydrogen shows three behaviors: lose (H⁺ in acids), gain (H⁻ in ionic hydrides), or share (covalent compounds) — unique among elements.
- Hydride classes: ionic (NaH, CaH₂), covalent (CH₄, H₂O, NH₃, HCl), metallic/interstitial (TiH₂).
- Laboratory: metal + acid, e.g., Zn + 2HCl → ZnCl2 + H2; only metals above H in the series work.
- Industrial: steam reforming CH4 + H2O → CO + 3H2 plus water-gas shift; electrolysis of water gives pure H₂.
- Combustion: 2H2 + O2 → 2H2O, very exothermic — rocket fuel, fuel cells, hydrogenation.
- H₂–air mixtures are explosive over a wide range; H₂ burns with a nearly invisible flame — ventilation and ignition control are essential.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Name the three isotopes of hydrogen, and give one property that makes each important.
Show answer
Protium (¹H) — the dominant, stable form; deuterium (²H) — stable, ~0.0156% abundant, makes heavy water D₂O used as a reactor moderator; tritium (³H) — radioactive β⁻ emitter (half-life ~12.3 yr), used in fusion research and luminous devices.
Write the reaction of sodium with hydrogen gas, and classify the product.
Show answer
2Na + H2 → 2NaH. Sodium hydride is an ionic hydride containing H⁻; it reacts vigorously with water to regenerate H₂.
Why is zinc (not copper) used in the classic laboratory preparation of hydrogen from acid?
Show answer
Zinc is above hydrogen in the activity series, so it displaces H₂ from acid at a convenient, controllable rate. Copper is below hydrogen and cannot displace it at all.
How many liters of H₂ at STP form when 13.0 g of Zn reacts with excess HCl? (Use Example 1's method.)
Show answer
n(Zn) = 13.0/65.38 = 0.1988 mol = n(H2); V = 0.1988 × 22.4 = 4.45 L.
What are the two main industrial routes to H₂, and why is one called "green"?
Show answer
Steam reforming of methane (CH4 + H2O → CO + 3H2, followed by the water-gas shift) — the dominant, fossil-based route; and electrolysis of water (2H2O → 2H2 + O2) — called "green" when powered by renewable electricity, since it produces H₂ without direct CO₂ emissions.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Isotope
- Same element, different number of neutrons
- Protium
- ¹H — one proton, no neutrons
- Deuterium
- ²H (D) — proton + neutron
- Tritium
- ³H (T) — proton + 2 neutrons, radioactive β⁻ emitter
- Hydride
- A compound of hydrogen with another element
- Steam reforming
- Industrial H₂ production from methane + steam
- Electrolysis of water
- Splitting H₂O into H₂ and O₂ with electricity
- Hydrogenation
- Adding H₂ across a double bond
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.