Helium is a chemical element with atomic number 2, a gas at nearly all temperatures and pressure conditions, with a low reactivity. It is used to fill balloons and airships and as a coolant for superconductors. It cannot exist as a solid or liquid at normal pressures.
Helium is a chemical element with atomic number 2, which means that a neutral helium atom has two protons and two electrons. The most important chemical properties of helium include its atomic mass, state of matter, boiling and melting points, and density. The element has an atomic mass of 4.0026 grams per mole and is a gas at nearly all temperatures and pressure conditions. The density of helium is 0.1786 grams per liter at 32°F (0.0°C) and 101.325 kilopascals (kPa).
Liquid and solid helium can only exist under conditions of high pressure at extremely low temperatures. One of the unusual properties of helium is that it cannot exist as a solid or liquid at normal pressures, even at extremely low temperatures. At a pressure of about 360 pounds per square inch (2.5 megapascals), the liquid-solid transition, or melting point, is -458°F (0.95 Kelvin). The boiling point is -452°F (4.22 Kelvin).
Some of the properties of helium make it an interesting and common subject of study in quantum mechanics. It is, due to its low atomic number, the second simplest atom after hydrogen. Mathematical procedures can be used to analyze the behaviors of subatomic particles – protons, electrons and neutrons – inside the helium atom. Such methods cannot, however, determine the behavior of these particles with absolute certainty. Atoms with larger atomic numbers, which have more subatomic particles, tend to be more difficult to work with in terms of quantum mechanical analysis.
Helium is the least reactive of all elements. The nonreactive properties of helium arise from the fact that it is the lightest of the generally unreactive noble gases. A noble gas has a “full” electron shell, which means it cannot easily give or receive electrons in a chemical reaction. The exchange or sharing of electrons underlies most chemical reactions, so the noble gases tend to participate in few chemical reactions. Furthermore, helium has only two electrons that could participate in a reaction, while all other noble gases—and indeed, all elements apart from hydrogen—have more.
There are many different uses for helium stemming from the chemical properties of helium, most notably its light weight, its temperature and pressure qualities, and its low reactivity. Helium is, for example, considerably lighter than air, so it’s often used to fill balloons so they can float, and airships like blimps so they can fly. Liquid helium, which can exist only at extreme pressures and very low temperatures, is used as a coolant for superconductors, which only take on their highly conductive properties at very low temperatures.
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