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| Helium |
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| Atomic Number - | 2 | Melting Point (°C,°F) - | -272.2 °C, -458.0 °F |
| Atomic Symbol - | He | Boiling Point (°C,°F) - | -268.93 °C, -452.07 °F |
| Atomic Mass - | 4.00260 | Electron Configuration - | 1s2 |
| Group - | 18 | Electrons Per Shell - | 2 |
| Period - | 1 | Protons - | 2 |
| Series - | Noble Gases | Neutrons - | 2 |
| Block - | d-block | | |
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Element Description - Helium is a chemical element that has the symbol He, is colorless, odorless, tasteless, non-toxic, and nearly inert monatomic that heads the noble gas series in the periodic table. Its atomic number is 2 and its boiling and melting points are the lowest among the elements. It exists only as a gas except in extreme conditions. Extreme conditions are also needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure. Its most abundant stable isotope is helium-4 and it has a rare stable isotope, helium-3. The behavior of liquid helium-4's two different states—helium I and helium II—is important to researchers studying quantum mechanics (in particular the phenomenon of superfluidity) and those looking at the effects that near absolute zero temperatures have on matter (such as superconductivity).
Helium is the second most abundant and second lightest element in the periodic table. In the modern Universe almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. On Earth it is created by the radioactive decay of much heavier elements (alpha particles are helium-4 nuclei produced by alpha-decay). After its creation, part of it is trapped with natural gas in concentrations up to 7% by volume. It is extracted from the natural gas by a low temperature separation process called fractional distillation. |
Element Characteristics - Helium is a colorless, odorless, and non-toxic gas. It is the least reactive member of group 18 (the noble gases) of the periodic table and is therefore virtually inert. Under standard temperature and pressure helium behaves very much like an ideal gas. Under virtually all conditions helium is monatomic. It has a thermal conductivity that is greater than any gas except hydrogen and its specific heat is unusually high. Helium is also less water soluble than any other gas known and its diffusion rate through solids is three times that of air and around 65% that of hydrogen. Helium's index of refraction is closer to unity than any other gas. This gas has a negative Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its Joule-Thomson inversion temperature (of about 40 K at 1 atmosphere) does it cool upon free expansion. Once precooled below this temperature, helium can be liquefied through expansion cooling.
Helium is chemically unreactive under all normal conditions due to its valence of zero. It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential. Helium can form unstable compounds with tungsten, iodine, fluorine, sulfur and phosphorus when it is subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma. HeNe, HgHe10, WHe2 and the molecular ions He2+, He2++, HeH+, and HeD+ have been created this way. This technique has also allowed the production of the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces . Theoretically, other compounds, like helium fluorohydride (HHeF), may also be possible.
Throughout the Universe, helium is found mostly in a plasma state whose properties are quite different to molecular helium. As a plasma, helium's electrons and protons are not bound together, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind together with ionized hydrogen, they interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora. |
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