Boc study guide 5th edition pdf download


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This article is about the chemical element. 12 times the mass of all the heavier elements combined. Helium is named for the Greek Titan of the Sun, Helios. On Earth it is relatively rare—5. 2 ppm by volume in the atmosphere.

The first evidence of helium was observed on August 18, 1868, as a bright yellow line with a wavelength of 587. 49 nanometers in the spectrum of the chromosphere of the Sun. Picture of visible spectrum with superimposed sharp yellow and blue and violet lines. In 1881, Italian physicist Luigi Palmieri detected helium on Earth for the first time through its D3 spectral line, when he analyzed a material that had been sublimated during a recent eruption of Mount Vesuvius. The cleveite sample from which Ramsay first purified helium.

In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei by allowing the particles to penetrate the thin glass wall of an evacuated tube, then creating a discharge in the tube to study the spectrum of the new gas inside. In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. This enabled the United States to become the world’s leading supplier of helium. Following a suggestion by Sir Richard Threlfall, the United States Navy sponsored three small experimental helium plants during World War I.

The goal was to supply barrage balloons with the non-flammable, lighter-than-air gas. Although the extraction process, using low-temperature gas liquefaction, was not developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craft. The government of the United States set up the National Helium Reserve in 1925 at Amarillo, Texas, with the goal of supplying military airships in time of war and commercial airships in peacetime.

Bureau of Mines arranged for five private plants to recover helium from natural gas. 4 billion in debt, prompting the Congress of the United States in 1996 to phase out the reserve. Helium produced between 1930 and 1945 was about 98. In 1945, a small amount of 99. By 1949, commercial quantities of Grade A 99. Canada, Poland, Russia, and other nations produced the remainder.

Meanwhile, by 2000, the consumption of helium within the U. As of 2012, the United States National Helium Reserve accounted for 30 percent of the world’s helium. The reserve was expected to run out of helium in 2018. In 2013, Qatar started up the world’s largest helium unit, although the 2017 Qatar diplomatic crisis severely affected helium production there. Picture of a diffuse gray sphere with grayscale density decreasing from the center. An inset outlines the structure of the core, with two red and two blue atoms at the length scale of 1 femtometer. 4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case.

In the perspective of quantum mechanics, helium is the second simplest atom to model, following the hydrogen atom. The nucleus of the helium-4 atom is identical with an alpha particle. High-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium’s own electron cloud. For example, the stability and low energy of the electron cloud state in helium accounts for the element’s chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements. In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions that involve either heavy-particle emission or fusion. Binding energy per nucleon of common isotopes. The binding energy per particle of helium-4 is significantly larger than all nearby nuclides.

The unusual stability of the helium-4 nucleus is also important cosmologically: it explains the fact that in the first few minutes after the Big Bang, as the “soup” of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nuclei. Big Bang in stars which were hot enough to fuse helium itself. Helium is the second least reactive noble gas after neon, and thus the second least reactive of all elements. It is chemically inert and monatomic in all standard conditions. CF4, SF6, and C4F8 have lower mole fraction solubilities: 0. Most extraterrestrial helium is found in a plasma state, with properties quite different from those of atomic helium.

In a plasma, helium’s electrons are not bound to its nucleus, 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. This helium is not only liquid, but has been cooled to the point of superfluidity. The drop of liquid at the bottom of the glass represents helium spontaneously escaping from the container over the side, to empty out of the container. The energy to drive this process is supplied by the potential energy of the falling helium.