Hydrogen can form compounds with elements that are more electronegative, such as halogens (F, Cl, Br, I), or oxygen; in these compounds hydrogen takes on a partial positive charge. 31 When bonded to a more electronegative element, particularly fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a lone pair, a phenomenon called hydrogen bonding that is critical to the stability of many biological molecules. 32 33 Hydrogen also forms compounds with less electronegative elements, such as metals and metalloids, where it takes on a partial negative charge.With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table.
Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75 of all baryonic mass. Non- remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium (name rarely used, symbol 1 H), has one proton and no neutrons. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas with the molecular formula H 2. Since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a particularly important role in acidbase reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge (i.e., anion ) when it is known as a hydride, or as a positively charged (i.e., cation ) species denoted by the symbol H. The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrdinger equation can be solved analytically, 8 study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. In 176681, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, 9 and that it produces water when burned, the property for which it was later named: in Greek, hydrogen means water-former. Hydrogen is problematic in metallurgy because it can embrittle many metals, 11 complicating the design of pipelines and storage tanks. When the bottom cap is removed, allowing air to enter at the bottom, the hydrogen in the container rises out of top and burns as it mixes with the air. The explosive reactions may be triggered by spark, heat, or sunlight. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames. The destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. The kinetic basis of the low reactivity is the nonpolar nature of H 2 and its weak polarizability. It spontaneously reacts with chlorine and fluorine to form hydrogen chloride and hydrogen fluoride, respectively. Molten sodium and potassium react with the gas to give the respective hydrides NaH and KH. Thus, while H 2 combusts readily, mixtures of H 2 and O 2 do not react in the absence of a catalyst. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. Because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at allillustrating how the planetary orbit differs from electron motion. The liquid and gas phase thermal properties of pure parahydrogen differ significantly from those of the normal form because of differences in rotational heat capacities, as discussed more fully in spin isomers of hydrogen. The orthopara distinction also occurs in other hydrogen-containing molecules or functional groups, such as water and methylene, but is of little significance for their thermal properties. Hydrogen can form compounds with elements that are more electronegative, such as halogens (F, Cl, Br, I), or oxygen; in these compounds hydrogen takes on a partial positive charge. When bonded to a more electronegative element, particularly fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a lone pair, a phenomenon called hydrogen bonding that is critical to the stability of many biological molecules. Hydrogen also forms compounds with less electronegative elements, such as metals and metalloids, where it takes on a partial negative charge.
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