Chemistry: Atoms First 2e · Representative Metals, Metalloids, and Nonmetals

Occurrence, Preparation, and Compounds of Hydrogen

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In 30 seconds
Why this matters
The college version
Eli explains
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Hydrogen is the simplest element — one proton, one electron — and the most abundant in the universe: about 75% of its ordinary matter by mass, roughly 90% of its atoms. On Earth the story differs: free H₂ is rare because it is reactive and so light it escapes the atmosphere. Nearly all terrestrial hydrogen is combined — in water, hydrocarbons, and every living molecule. The crust is only ~0.15% hydrogen by mass, but because of water it ranks among the most abundant elements by atom count.

Hydrogen's chemistry is unique. Its single electron can be lost (forming the bare proton H⁺), gained (forming the ion H⁻, like the halogens), or shared (forming covalent bonds, like carbon). That triple personality makes hydrogen a fuel, a chemical feedstock, and the key to acid–base chemistry. This topic covers where it comes from, how it is prepared, and the compound families — hydrides — it forms.

Why this matters

Energy future: hydrogen burns to water with no CO₂, and fuel cells power vehicles and backup systems. "Green hydrogen" hinges on powered by renewables.
Fertilizer and food: roughly half of the world's food production depends on ammonia made from hydrogen and nitrogen (Haber process).
Your body runs on hydrogen chemistry: water, every C–H bond in fats and sugars, and the pH scale all involve hydrogen.
Exam value: expect questions on hydrogen's isotopes, the metal + acid preparation, hydride classification (ionic vs. covalent), and the acidity trend of binary hydrides.

The college version

Core Concepts

Occurrence and the three isotopes

All three isotopes share one proton:

(¹H): one proton, no neutrons — 99.985% of natural hydrogen.
(²H or D): one proton, one neutron — about 0.015% of natural hydrogen; twice the mass of protium.
(³H or T): one proton, two neutrons — radioactive, with a half-life of about 12.3 years; only trace amounts exist naturally.

Because hydrogen isotopes differ by 100% in mass — far more than isotopes of other elements — deuterium substitution measurably changes reaction rates and physical properties. "Heavy water" (D₂O) is denser than ordinary water and moderates neutrons in some nuclear reactors.

Preparation: three classic routes

Lab — metal + acid. An active metal (above hydrogen in the activity series) displaces hydrogen from a dilute acid:

$Zn (s) + 2 HCl (aq) ⟶ ZnCl 2 (aq) + H 2 (g)$

Metals below hydrogen (Cu, Ag, Au) do not react with dilute acid — a quick reactivity test. Active metals + water is even more vigorous: sodium reacts violently with water, so this route is never used casually. (General safety principle: any experiment generating hydrogen needs ventilation and no open flames, because H₂ forms explosive mixtures with air.)

Industrial — of methane. Most commercial hydrogen comes from natural gas:

$CH 4 (g) + H 2 O (g) ⟶ CO (g) + 3 H 2 (g)$

The older water-gas reaction uses coke and steam: C(s) + H₂O(g) ⟶ CO(g) + H₂(g). Both routes give "syngas" (CO + H₂), which is shifted and purified.

Electrolysis of water. Passing a direct current through water (with a little electrolyte) splits it cleanly:

$2 H 2 O (l) ⟶ 2 H 2 (g) + O 2 (g)$

Electrolysis gives the purest hydrogen but costs electricity, so it is used when purity matters (electronics, hydrogenation catalysts) or when renewable power is cheap.

Compounds: the three faces of hydrogen

Ionic (salt-like) hydrides. With very active metals, hydrogen gains an electron to become H⁻: NaH, CaH₂, LiH. These white, crystalline, high-melting solids react with water to release H₂: NaH + H₂O ⟶ NaOH + H₂. The hydride ion is a powerful reducing agent and base.

Covalent hydrides. With nonmetals, hydrogen shares its electron: CH₄, NH₃, H₂O, HCl, H₂S. These molecular substances have low boiling points, and hydrogen holds oxidation state +1. Across the second row, the binary hydrides show a beautiful trend:

CH₄ — no acidic or basic character.
NH₃ — a weak base.
H₂O — neutral, amphoteric.
HF — a weak acid.

Down a group the pattern flips: among the hydrogen halides, acidity increases, HF < HCl < HBr < HI, because the H–X bond weakens faster than ion stability changes. The opposite of what electronegativity alone suggests — a classic exam trap.

Metallic (interstitial) hydrides. Many transition metals absorb hydrogen into the spaces between atoms (e.g., PdH₀.₆), storing it compactly for hydrogen-storage and battery research.

Hydrogen as a reducing agent

Hydrogen gas reduces many metal oxides at elevated temperature:

$CuO (s) + H 2 (g) ⟶ Cu (s) + H 2 O (g)$

This is the reverse of how hydrogen is made from water — the reaction's direction depends entirely on conditions. Attached to N, O, or F, hydrogen also forms hydrogen bonds, the interaction behind water's high boiling point and DNA's double helix.

Examples: hydrogen stoichiometry

Example 1 — Lab preparation: how much zinc for a liter of hydrogen?

A student wants 1.00 L of H₂ at STP (1 mol gas = 22.4 L) from Zn + 2HCl ⟶ ZnCl₂ + H₂. Moles of hydrogen:

$n(H 2) = 1.00 L 22.4 L/mol = 4.46 \times 10 -2 mol$

The balanced equation gives a 1:1 Zn : H₂ ratio, so:

$m(Zn) = 4.46 \times 10 -2 mol H 2 \times 1 mol Zn 1 mol H 2 \times 65.38 g Zn 1 mol Zn = 2.92 g$

About 2.92 g of zinc — with 8.92×10⁻² mol (≈3.25 g) of HCl — produces one liter of hydrogen.

Example 2 — Heavy water: why D₂O is "heavy"

Protium is 1.008 g/mol; deuterium is 2.014 g/mol:

$M(H 2 O) = 2(1.008) + 16.00 = 18.02 g/mol$

$M(D 2 O) = 2(2.014) + 16.00 = 20.03 g/mol$

Percent difference:

$20.03 - 18.02 18.02 \times 100 % = 11.2 %$

Deuterium substitution makes the molecule about 11% heavier — enough to change density, freezing point, and biological reaction rates, which is why D₂O in bulk is toxic to organisms despite being chemically "just water."

Example 3 — Steam reforming: how much hydrogen from 16.0 g of methane?

Using CH₄ + H₂O ⟶ CO + 3H₂:

$n(CH 4) = 16.0 g 16.04 g/mol = 0.998 mol$

Mole ratio 1 CH₄ : 3 H₂, then convert moles to volume at STP:

$V(H 2) = 0.998 mol CH 4 \times 3 mol H 2 1 mol CH 4 \times 22.4 L 1 mol H 2 = 67.1 L$

One mole of methane yields about 67 L of H₂ at STP — the high hydrogen yield per carbon is why steam reforming dominates industry.

Common Confusions

Do Not Confuse	With	Difference
H⁺ (proton)	H⁻ (hydride)	H⁺ has lost its electron; H⁻ has gained one. They are chemical opposites (acid vs. powerful base).
Hydrogen (nonmetal)	Group 1 metals	H sits in group 1 but is a nonmetal: it gains an electron (H⁻) as easily as it loses one, and H₂ is a gas, not a solid metal.
Deuterium	Tritium	Both are heavy isotopes; deuterium is stable, tritium is radioactive (t½ ≈ 12.3 y).
Any metal + any acid	Active metal + dilute acid	Only metals above hydrogen in the activity series produce H₂; Cu, Ag, Au do not react with dilute acids.
HF acidity	HCl, HBr, HI acidity	HF is a weak acid despite fluorine's huge electronegativity; HCl, HBr, HI are strong acids. Down the group, acidity increases.
Water gas (CO + H₂)	Water vapor (H₂O gas)	Water gas is a fuel/synthesis mixture; water vapor is simply gaseous water.
Hydrogen bonding	Covalent bond to H	A hydrogen bond is a weak intermolecular attraction (e.g., between water molecules); the O–H covalent bond is strong and intramolecular.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Hydrogen is the simplest element — just one proton and one electron — and it is the most common thing in the universe, the fuel of stars. On Earth it is almost always hiding inside other stuff, like water. It is a social element: it can give away its electron, grab an extra one, or share, which is why it makes acids, fuels rockets, and is a key part of every living thing.

Key takeaways

Hydrogen is the most abundant element in the universe (~75% by mass) but rare as free H₂ on Earth; it is found combined in water and hydrocarbons.
Three isotopes: ¹H protium (99.985%), ²H deuterium (0.015%), ³H tritium (radioactive, t½ ≈ 12.3 y).
Preparations: Zn + 2HCl → ZnCl₂ + H₂ (lab); CH₄ + H₂O → CO + 3H₂ (steam reforming); 2H₂O → 2H₂ + O₂ (electrolysis).
Only metals above hydrogen in the activity series displace H₂ from dilute acids; Cu, Ag, Au do not.
Three hydride families: ionic (NaH, CaH₂ — H is −1), covalent (CH₄, NH₃, H₂O, HCl — H is +1), metallic/interstitial (PdH₀.₆).
Binary-hydride acidity: across period 2, CH₄ (neutral) < NH₃ (weak base) < H₂O (amphoteric) < HF (weak acid); down group 17, HF < HCl < HBr < HI.
H₂ is a reducing agent: CuO + H₂ → Cu + H₂O.
Safety: H₂–air mixtures are explosive; always generate hydrogen with ventilation and no ignition sources.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Name hydrogen's three isotopes, with their neutron counts and natural abundances.
Show answer
Protium ¹H (0 neutrons, 99.985%); deuterium ²H (1 neutron, ~0.015%); tritium ³H (2 neutrons, trace, radioactive).
Write the balanced equation for preparing hydrogen from zinc and hydrochloric acid.
Show answer
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g).
Which hydrides contain H⁻ — ionic or covalent? Give one example.
Show answer
Ionic (salt-like) hydrides of active metals contain H⁻, e.g., NaH or CaH₂. Covalent hydrides (CH₄, NH₃, H₂O, HCl) contain H in the +1 state.
Arrange HF, HCl, HBr, HI in order of increasing acid strength.
Show answer
HF < HCl < HBr < HI.
How many grams of zinc produce 5.60 L of H₂ at STP? (M: Zn = 65.38 g/mol; 1 mol gas = 22.4 L at STP.)
Show answer
n(H₂) = 5.60 L ÷ 22.4 L/mol = 0.250 mol = n(Zn); m(Zn) = 0.250 × 65.38 = 16.3 g.
Why is electrolysis used for hydrogen when steam reforming is cheaper?
Show answer
Electrolysis produces ultrapure hydrogen without CO₂ co-product (when powered by renewable electricity), which matters for fuel cells, electronics, and catalysts; steam reforming is cheaper but emits CO₂ and leaves CO impurities.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Protium: ¹H — hydrogen with no neutrons (99.985%).
Deuterium: ²H — hydrogen with one neutron; "heavy hydrogen."
Tritium: ³H — radioactive hydrogen with two neutrons.
Hydride: A compound of hydrogen with another element.
Ionic hydride: Salt-like hydride of an active metal (NaH, CaH₂); H is −1.
Covalent hydride: Molecular compound of H and a nonmetal (CH₄, NH₃, H₂O, HCl).
Steam reforming: CH₄ + H₂O → CO + 3H₂ at high temperature.
Electrolysis: Splitting water into H₂ and O₂ with electric current.
Water gas: The CO + H₂ mixture from C + H₂O.

Sources & references

openstax.org — Chemistry Atoms First 2e

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