Facts About Hydrogen

Atomic Number: 1
Atomic Symbol: H
Atomic Weight: 1.00794
Melting Point: -434.7 F (-259.34 C)
Boiling Point: -423.2 F (-252.87 C)

Word origin: The word hydrogen comes from the Greek word hydro (water) and genes (forming).

Discovery: Hydrogen was recognized as a distinct substance by Henry Cavendish in 1776.

Properties of hydrogen

Hydrogen is the most abundant element in the universe. The heavier elements were originally made from hydrogen atoms or from other elements that were originally made from hydrogen atoms. [See Periodic Table of the Elements]

Hydrogen is estimated to make up more than 90 percent of all the atoms — three quarters of the mass of the universe. This element plays an important part in powering the universe through both the proton-proton reaction and carbon-nitrogen cycle. Stellar hydrogen fusion processes release massive amounts of energy by combining hydrogen to form helium.

This true-color simulated view of Jupiter is composed of 4 images taken by NASA's Cassini spacecraft on December 7, 2000. The resolution is about 89 miles (144 kilometers) per pixel.
Credit: NASA/JPL/University of Arizona

Hydrogen is the primary component of Jupiter and the other gas giant planets. At some depth in the planet's interior the pressure is so great that solid molecular hydrogen is converted to solid metallic hydrogen.

Although pure hydrogen is a gas, there is very little of it in Earth's atmosphere. Hydrogen gas is so light that, uncombined, hydrogen will gain enough velocity from collisions with other gases that they will quickly be ejected from the atmosphere.

On Earth, hydrogen occurs chiefly in combination with oxygen in water, but it is also present in organic matter such as living plants, petroleum and coal. It is present as the free element in the atmosphere, but only less than 1 parts per million (ppm) by volume. The lightest of all gases, hydrogen combines with other elements — sometimes explosively — to form compounds.

Uses of hydrogen

About 3 billion cubic feet of hydrogen is produced per year in the United States. Hydrogen is prepared by:

  • steam on heated carbon
  • decomposition of certain hydrocarbons with heat
  • reaction of sodium or potassium hydroxide on aluminum
  • electrolysis of water
  • displacement from acids by certain metals

Great quantities of hydrogen are required commercially for producing fertilizer and for hydrogenating fats and oils. Large quantities of hydrogen are used in methanol production and in refining petroleum. Other uses include rocket fuel, welding, producing hydrochloric acid, reducing metallic ores and filling balloons.

Liquid hydrogen is important in cryogenics and in the study of superconductivity, as its melting point is only 20 degrees above absolute zero.

Diagram of the fuel cell and hydrogen tanks in the Chevy Equinox.
Credit: General Motors.

The hydrogen fuel cell is a developing technology that will allow great amounts of electrical power to be obtained using a source of hydrogen gas. [Countdown: Top 10 Emerging Environmental Technologies]

Isotopes of hydrogen

The ordinary isotope of hydrogen is known as protium. Hydrogen has two other isotopes: deuterium and tritium. Hydrogen is the only element whose isotopes have been given different names. Deuterium and tritium are both used as fuel in nuclear fusion reactors. One atom of deuterium is found in about 6,000 ordinary hydrogen atoms.

Deuterium is used as a moderator to slow down neutrons. Tritium atoms are also present but in much smaller proportions. Tritium is readily produced in nuclear reactors and is used in the production of the hydrogen (fusion) bomb. It is also used as a radioactive agent in making luminous paints, and as a tracer.

(Source: Los Alamos National Laboratory)

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