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Hydrogen - H

Chemical properties of hydrogen - Health effects of hydrogen - Environmental effects of hydrogen

Atomic number


Atomic mass

1.007825 g.mol -1

Electronegativity according to Pauling



0.0899*10 -3 g.cm -3 at 20 °C

Melting point

- 259.2 °C

Boiling point

- 252.8 °C

Vanderwaals radius

0.12 nm

Ionic radius

0.208 (-1) nm



Electronic shell


Energy of first ionisation

1311 kJ.mol -1

Discovered by

Henry Cavendish in 1766*

* Hydrogen was observed and collected long before it was recognized as a unique gas by Robert Boyle in 1671, who dissolved iron in diluted hydrochloric acid.
source: homepage.mac.com/dtrapp/Elements/properties.html

Hydrogen - H


First element in the periodic table. In normal conditions it’s a colourless, odourless and insipid gas, formed by diatomic molecules, H2. The hydrogen atom, symbol H, is formed by a nucleus with one unit of positive charge and one electron. Its atomic number is 1 and its atomic weight 1,00797 g/mol. It’s one of the main compounds of water and of all organic matter, and it’s widely spread not only in The Earth but also in the entire Universe. There are three hydrogen isotopes: protium, mass 1, found in more than 99,985% of the natural element; deuterium, mass 2, found in nature in 0.015% approximately, and tritium, mass 3, which appears in small quantities in nature, but can be artificially produced by various nuclear reactions.

Uses: The most important use of hydrogen is the ammonia synthesis. The use of hydrogen is extending quickly in fuel refinement, like the breaking down by hydrogen (hydrocracking), and in sulphur elimination. Huge quantities of hydrogen are consumed in the catalytic hydrogenation of unsaturated vegetable oils to obtain solid fat. Hydrogenation is used in the manufacture of organic chemical products. Huge quantities of hydrogen are used as rocket fuels, in combination with oxygen or fluor, and as a rocket propellent propelled by nuclear energy.
Hydrogen can be burned in internal combustion engines. Hydrogen fuel cells are being looked into as a way to provide power and research is being conducted on hydrogen as a possible major future fuel. For instance it can be converted to and from electricity from bio-fuels, from and into natural gas and diesel fuel, theoretically with no emissions of either CO2 or toxic chemicals.

Properties: Common hydrogen has a molecular weight of 2,01594 g. As a gas it has a density of 0.071 g/l at 0ºC and 1 atm. Its relative density, compared with that of the air, is 0.0695. Hydrogen is the most flammable of all the known substances. Hydrogen is slightly more soluble in organic solvents than in water. Many metals absorb hydrogen. Hydrogen absorption by steel can result in brittle steel, which leads to fails in the chemical process equipment.

At normal temperature hydrogen is a not very reactive substance, unless it has been activated somehow; for instance, by an appropriate catalyser. At high temperatures it’s highly reactive.

Although in general it’s diatomic, molecular hydrogen dissociates into free atoms at high temperatures. Atomic hydrogen is a powerful reductive agent, even at ambient temperature. It reacts with the oxides and chlorides of many metals, like silver, copper, lead, bismuth and mercury, to produce free metals. It reduces some salts to their metallic state, like nitrates, nitrites and sodium and potassium cyanide. It reacts with a number of elements, metals and non-metals, to produce hydrides, like NAH, KH, H2S and PH3. Atomic hydrogen produces hydrogen peroxide, H2O2, with oxygen.

Atomic hydrogen reacts with organic compounds to form a complex mixture of products; with etilene, C2H4, for instance, the products are ethane, C2H6, and butane, C4H10. The heat released when the hydrogen atoms recombine to form the hydrogen molecules is used to obtain high temperatures in atomic hydrogen welding.

Hydrogen reacts with oxygen to form water and this reaction is extraordinarily slow at ambient temperature; but if it’s accelerated by a catalyser, like platinum, or an electric spark, it’s made with explosive violence.

Health effects of hydrogen

Effects of exposure to hydrogen: Fire: Extremely flammable. Many reactions may cause fire or explosion. Explosion: Gas/air mixtures are explosive. Routes of exposure: The substance can be absorbed into the body by inhalation. Inhalation: High concentrations of this gas can cause an oxygen-deficient environment. Individuals breathing such an atmosphere may experience symptoms which include headaches, ringing in ears, dizziness, drowsiness, unconsciousness, nausea, vomiting and depression of all the senses. The skin of a victim may have a blue color. Under some circumstances, death may occur. Hydrogen is not expected to cause mutagenicity, embryotoxicity, teratogenicity or reproductive toxicity. Pre-existing respiratory conditions may be aggravated by overexposure to hydrogen. Inhalation risk: On loss of containment, a harmful concentration of this gas in the air will be reached very quickly.

Physical dangers: The gas mixes well with air, explosive mixtures are easily formed. The gas is lighter than air.

Chemical dangers: Heating may cause violent combustion or explosion. Reacts violently with air, oxygen, halogens and strong oxidants causing fire and explosion hazard. Metal catalysts, such as platinum and nickel, greatly enhance these reactions.

High concentrations in the air cause a deficiency of oxygen with the risk of unconsciousness or death. Check oxygen content before entering area. No odor warning if toxic concentrations are present. Measure hydrogen concentrations with suitable gas detector (a normal flammable gas detector is not suited for the purpose).

First aid: Fire: Shut off supply; if not possible and no risk to surroundings, let the fire burn itself out; in other cases extinguish with water spray, powder, carbon dioxide. Explosion: In case of fire: keep cylinder cool by spraying with water. Combat fire from a sheltered position. Inhalation: Fresh air, rest. Artificial respiration may be needed. Refer for medical attention. Skin: Refer for medical attention.

Environmental effects of hydrogen

Hydrogen in the environment: Hydrogen forms 0.15 % of the earth's crust, it is the major constituent of water. 0.5 ppm of hydrogen H2 and varial proportions as water vapour are present in the atmosphere. Hydrogen is also a majosr component of biomass, consituing the 14% by weight.

Environmental stability: hydrogen occurs naturally in the atmosphere. The gas will be dissipated rapidly in well-ventilated areas.

Effect on plants or animals: Any effect on animals would be related to oxygen deficient environments. No adverse effect is anticipated to occur to plant life, except for frost produced in the presence of rapidly expanding gases.

Effect on aquatic life: No evidence is currently available on the effect of hydrogen on aquatic life.

Back to the periodic table of elements.

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