Helium
Helium is the first member of the noble gas family, the elements that make up Group 18 of the periodic table. Its atomic number is 2, its atomic mass is 4.002602, and its chemical symbol is He.
Properties
Helium is a colorless, odorless, tasteless gas with a number of unusual properties. For example, it has the lowest boiling point of any element, -452°F (-268.9°C). Its freezing point is -458°F (-272.2°C). At a temperature of about -456°F (-271°C), helium undergoes a striking change. It remains a liquid, but a liquid with unusual properties. One of the most interesting of those properties is superfluidity, a condition in which a liquid flows upwards out of a container against the force of gravity and may pass through very small holes that would normally be impermeable to the gas. Chemically, helium is completely inert. It does not form compounds or react with any other element.
Occurrence and Extraction
Helium is the second most abundant element after hydrogen in the universe and in the solar system. About 11.3% of all atoms in the universe are helium atoms. By contrast, about 88.6% of all atoms in the universe are hydrogen. Helium is much less abundant on Earth. It is the sixth most abundant gas in the atmosphere after nitrogen, oxygen, argon, carbon dioxide, and neon. It makes up about 0.000524% of the air. Helium is also formed in the Earth's crust during the radioactive decay of uranium and other radioactive elements, but it usually escapes into the atmosphere soon after being formed.
Discovery and Naming
Helium was first discovered not on Earth, but in the Sun. In 1868, the French astronomer Pierre Janssen studied light from the Sun during a solar eclipse using a spectroscope. He found spectral lines that had never been observed before and decided that they were produced by a new element. He suggested the name helium for the element after the Greek word for the Sun, helios. Helium was first discovered on Earth in 1895 by the English physicist Sir William Ramsay who found the element in a mineral containing the element uranium.
Uses
The most important single use for helium in the United States is in low-temperature cooling systems. Since helium can be cooled to a lower temperature than any other material, it can be used to cool any other material. Helium is also used to provide inert atmospheres for industrial operations. For example, it is used in welding systems to prevent the metals being heated from reacting with oxygen in the air before they join to each other. Helium is also used to purge and pressurize systems. Any gas could, in theory, be used for these purposes, but helium has the advantage of not reacting with any of the components of the system being purged or pressurized. One of helium's better-known, but less commercially important, applications is in lighter-than-air craft, such as dirigibles, and in weather and research balloons. For most purposes, helium has largely replaced hydrogen, which has a lower density but is more dangerous to work with.
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