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Hydrogen gas is very rare in Earth's atmosphere (around 0.53 ppm on a molar basis [99]) because of its light weight, which enables it to escape the atmosphere more rapidly than heavier gases. However, hydrogen is the third most abundant element on the Earth's surface, [100] mostly in the form of chemical compounds such as hydrocarbons and water ...
Hydrogen fluoride is lighter than air and could theoretically be used as a lifting gas. However, it is extremely corrosive, highly toxic, expensive, is heavier than other lifting gases, and has a low boiling point of 19.5 °C. Its use would therefore be impractical.
Because of its flammability, liquid hydrogen should be kept away from heat or flame unless ignition is intended. Unlike ambient-temperature gaseous hydrogen, which is lighter than air, hydrogen recently vaporized from liquid is so cold that it is heavier than air and can form flammable heavier-than-air air–hydrogen mixtures.
Since hydrogen is a lighter-than-air gas, it collects under roofs and overhangs (typically referred to as trapping sites), where it forms an explosion hazard. [14] Many individuals are familiar with protecting plants from heavier-than-air vapors, but are unfamiliar with "looking up", and is therefore of particular note. [33]
Heavier elements were mostly produced much later, in stellar nucleosynthesis. Hydrogen and helium are estimated to make up roughly 74% and 24% of all baryonic matter in the universe respectively. Despite comprising only a very small fraction of the universe, the remaining "heavy elements" can greatly influence astronomical phenomena.
Separately pressurised into convenient 'tanks' or 'gas bottles', hydrogen can be used for oxyhydrogen welding and other applications, as the hydrogen / oxygen flame can reach approximately 2,800°C. Water electrolysis requires a minimum potential difference of 1.23 volts , although at that voltage external heat is also required.
Water molecules have two hydrogen atoms and one oxygen atom. While H 2 is not very reactive under standard conditions, it does form compounds with most elements. 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. [1]
The amount of liquid-vapor equilibrium fractionation for hydrogen isotopes is about 8x that of oxygen isotopes at Earth surface temperatures, which reflects the relative mass differences of the two isotope systems: 2 H is 100% heavier than 1 H, 18 O is 12.5% heavier than 16 O. Above the boundary layer, there is a transition zone with relative ...