Hydrogen (Z = 1, configuration 1s1) is the lightest and most abundant element in the universe. Although it has just one electron, it refuses to sit comfortably in any single group of the periodic table.
Dual position in the periodic table
Like the alkali metals (Group 1) it has one valence electron (ns1), forms a unipositive ion H+, and acts as a reducing agent. Like the halogens (Group 17) it is one electron short of a noble-gas configuration, exists as a diatomic molecule H2, has high ionisation enthalpy and can gain an electron to form the hydride ion H−. Because it fits neither group perfectly, hydrogen is given a unique position at the top of the table.
Isotopes of hydrogen
Hydrogen has three isotopes, all with one proton but differing in neutrons:
- Protium (11H): no neutron, ≈ 99.985%, ordinary hydrogen.
- Deuterium (21H or D): one neutron, ≈ 0.015%, found in heavy water D2O.
- Tritium (31H or T): two neutrons, radioactive (β-emitter, half-life ≈ 12.3 years), present in traces.
Since they have identical electronic configurations, the isotopes show the same chemical behaviour but differ in rate of reaction (deuterium reacts more slowly — the kinetic isotope effect) and in physical properties such as density and boiling point.
Occurrence
In the free state hydrogen is rare on Earth (the atmosphere holds only traces) because it is too light to be held by gravity. In the combined state it is everywhere — chiefly as water (H2O), and in all acids, bases, hydrocarbons and living matter.
Methods of preparation
Laboratory: by the action of dilute sulphuric acid on granulated zinc — Zn + H2SO4 → ZnSO4 + H2. Very pure hydrogen comes from the electrolysis of acidified or barium-hydroxide water.
Commercial: (i) Electrolysis of warm acidified water between platinum electrodes gives H2 at the cathode. (ii) Water gas / syngas: passing steam over red-hot coke at about 1270 K gives a mixture of CO and H2 called water gas — C + H2O → CO + H2. In the water-gas shift reaction the CO is removed by reacting it with more steam over an iron-chromate catalyst: CO + H2O → CO2 + H2; the CO2 is scrubbed out under pressure to leave pure dihydrogen.
Properties
Physical: colourless, odourless, highly combustible gas; the lightest known substance; sparingly soluble in water. Chemical: the H–H bond is strong (435.9 kJ mol−1), so hydrogen is fairly unreactive cold but reactive when heated — it burns in oxygen to water, combines with halogens, reduces hot metal oxides, and hydrogenates unsaturated oils over nickel.
Give two properties of hydrogen that resemble the alkali metals and two that resemble the halogens.
Solution- Like alkali metals: it has a single valence electron (ns1) and forms a unipositive ion H+; it also acts as a reducing agent.
- Like halogens: it is one electron short of a noble-gas configuration and exists as a diatomic molecule H2; it can gain an electron to form the hydride ion H− and has a high ionisation enthalpy.
- Because it shares features of both, it is given a unique position in the periodic table.
Answer: Alkali-like: one valence electron / forms H+. Halogen-like: diatomic molecule / forms H−.
Name the three isotopes of hydrogen and state the number of neutrons in each.
Solution- Protium (11H): mass number 1, so neutrons = 1 − 1 = 0.
- Deuterium (21H): mass number 2, so neutrons = 2 − 1 = 1.
- Tritium (31H): mass number 3, so neutrons = 3 − 1 = 2.
Answer: Protium 0 neutrons, deuterium 1 neutron, tritium 2 neutrons.
Write the reactions involved in the manufacture of dihydrogen from water gas, including the shift reaction.
Solution- Pass steam over red-hot coke at about 1270 K to form water gas: C + H2O → CO + H2.
- Carry out the water-gas shift reaction over an iron-chromate catalyst: CO + H2O → CO2 + H2.
- Remove CO2 by scrubbing under pressure to obtain pure dihydrogen.
Answer: C + H2O → CO + H2; then CO + H2O → CO2 + H2.
Why is deuterium chemically similar to but kinetically slower than protium?
Solution- Both isotopes have the same electronic configuration (1s1), so they show the same chemical reactions.
- Deuterium has double the mass of protium, so the D–X bond is stronger and harder to break.
- This raises the activation energy and lowers the reaction rate — the kinetic isotope effect.
Answer: Identical electron configurations give identical chemistry, but the heavier D atom forms stronger bonds, so reactions of deuterium are slower.
Write the balanced equation for the laboratory preparation of dihydrogen and name the gas-collection method used.
Solution- React granulated zinc with dilute sulphuric acid: Zn + H2SO4 → ZnSO4 + H2.
- The gas evolved is dihydrogen.
- Because hydrogen is insoluble in water and lighter than air, it is collected by downward displacement of water.
Answer: Zn + H2SO4 → ZnSO4 + H2; collected over water (downward displacement of water).
The H–H bond enthalpy is 435.9 kJ mol−1. Explain how this accounts for the low reactivity of dihydrogen at room temperature.
Solution- A high bond enthalpy means a large amount of energy is needed to break the H–H bond into atoms.
- At room temperature few molecules have this much energy, so reactions are very slow.
- On heating or with a catalyst the bond can be broken, so hydrogen becomes reactive at high temperatures.
Answer: The strong H–H bond (high dissociation enthalpy) makes hydrogen unreactive at room temperature, requiring heat or a catalyst to react.