Hydrogen Facts - Element 1 or H

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Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.

Hydrogen (element symbol H and atomic number 1) is the first element on the periodic table and the most abundant element in the universe. Under ordinary conditions, it is a colorless flammable gas. This is a fact sheet for the element hydrogen, including its characteristics and physical properties, uses, sources and other data.

Essential Hydrogen Facts

Element Name: Hydrogen
Element Symbol: H
Element Number: 1
Element Category: nonmetal
Atomic Weight: 1.00794(7)
Electron Configuration: 1s 1
Discovery: Henry Cavendish, 1766. Cavendish prepared hydrogen by reacting metal with acid. Hydrogen was prepared for many years before it was recognized as a distinct element.
Word Origin: Greek: hydro meaning water; genes meaning forming. The element was named by Lavoisier.

Hydrogen Physical Properties

Phase (@STP): gas (Metallic hydrogen is possible under extremely high pressure.)
Appearance: Colorless, odorless, non-toxic, nonmetallic, flavorless, flammable gas.
Density: 0.89888 g/L (0°C, 101.325 kPa)
Melting Point: 14.01 K, -259.14 °C, -423.45 °F
Boiling Point: 20.28 K, -252.87 °C, -423.17 °F
Triple Point: 13.8033 K (-259°C), 7.042 kPa
Critical Point: 32.97 K, 1.293 MPa
Heat of Fusion: (H₂) 0.117 kJ·mol −1
Heat of Vaporization: (H₂) 0.904 kJ·mol −1
Molar Heat Capacity: (H₂) 28.836 J·mol−1·K −1
Ground Level: 2S_1/2
Ionization Potential: 13.5984 ev

Additional Hydrogen Properties

Specific Heat: 14.304 J/g•K

Hydrogen Sources

Free elemental hydrogen is found in volcanic gases and some natural gases. Hydrogen is prepared by decomposition of hydrocarbons with heat, action of sodium hydroxide or potassium hydroxide on aluminum electrolysis of water, steam on heated carbon, or displacement from acids by metals. Most hydrogen is used near the site of its extraction.

Hydrogen Abundance

Hydrogen is the most abundant element in the universe. The heavier elements formed from hydrogen or from other elements that were made from hydrogen. Although approximately 75% of the universe's elemental mass is hydrogen, the element is relatively rare on Earth. The element readily forms chemical bonds to be incorporated into compounds, however, the diatomic gas can escape Earth's gravity.

Hydrogen Uses

Commercially, most hydrogen is used to process fossil fuels and synthesize ammonia. Hydrogen is used in welding, hydrogenation of fats and oils, methanol production, hydrodealkylation, hydrocracking, and hydrodesulfurization. It is used to prepare rocket fuel, fill balloons, make fuel cells, make hydrochloric acid, and reduce metal ores. Hydrogen is important in the proton-proton reaction and carbon-nitrogen cycle. Liquid hydrogen is used in cryogenics and superconductivity. Deuterium is used as a tracer and a moderator to slow neutrons. Tritium is used in the hydrogen (fusion) bomb. Tritium is also used in luminous paints and as a tracer.

Hydrogen Isotopes

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The three naturally occurring isotopes of hydrogen have their own names: protium (0 neutrons), deuterium (1 neutron), and tritium (2 neutrons). In fact, hydrogen is the only element with names for its common isotopes. Protium is the most abundant hydrogen isotope, accounting for about 75 percent of the mass of the universe. 4 H to 7 H are extremely unstable isotopes that have been made in the lab but are not seen in nature.

Protium and deuterium are not radioactive. Tritium, however, decays into helium-3 through beta decay.

More Hydrogen Facts

• Hydrogen is the lightest element. Hydrogen gas is so light and diffusive that uncombined hydrogen can escape from the atmosphere.
• While pure hydrogen under ordinary conditions is a gas, other phases of hydrogen are possible. These include liquid hydrogen, slush hydrogen, solid hydrogen, and metallic hydrogen. Slush hydrogen is essentially a hydrogen slushie, containing bother the liquid at solid forms of the element at its triple point.
• Hydrogen gas is a mixture of two molecular forms, ortho- and para-hydrogen, which differ by the spins of their electrons and nuclei. Normal hydrogen at room temperature consists of 25% para-hydrogen and 75% ortho-hydrogen. The ortho form cannot be prepared in the pure state. The two forms of hydrogen differ in energy, so their physical properties also differ.
• Hydrogen gas is extremely flammable.
• Hydrogen can take a negative charge (H - ) or a positive charge (H + ) in compounds. Hydrogen compounds are called hydrides.
• Ionized deuterium displays a characteristic reddish or pink glow.
• Life and organic chemistry depend as much on hydrogen as on carbon. Organic compounds always contain both elements and the carbon-hydrogen bond gives these molecules their characteristic properties.