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# Helium

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A chemical element, in the periodic table helium has the symbol He and atomic number 2.

The second most abundant? element in the Universe. The commonest form has two protons and two neutrons. There is another isotope, 3He, which has only one neutron, as well as several heavier isotopes that are radioactive.

Helium is the first of the noble gases, and exists on earth as a monatomic gas.

It was first discovered by spectroscopy, when an emission line of a previously unknown element was observed in light from the Sun. Hence the name, derived from "helios", meaning "sun". Because, until its discovery on earth, it was thought that it was probably a metal, the name ends with "ium" which is the standard name ending for a metal.

The nucleus of the 4He atom is also known as alpha particle and is particularly stable (cf. [magic numbers]?).

When cooled to nearly absolute zero, Helium exhibits some unusual characteristics; for example, it does not become solid at reasonable pressures but on the contrary becomes superfluid?, which has historically been one of the first macroscopic quantum effects to be discovered (cf. Superconductivity).

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Last edited July 30, 2001 7:26 am (diff)
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