# General Chemistry/Chemistries of Various Elements/Hydrogen

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## Hydrogen

Hydrogen is by far the most common element in the universe; as a gas it is too light for Earth's gravity to hold. It is by far the largest constituent of the Sun and all other stars and of the gas giant planets of our solar system. It exists on or just under the surface of the Earth as a component of water and in innumerable compounds of carbon, many essential to life.
The heat and light from the sun (or any other star) arises largely from the nuclear fusion of hydrogen into helium. Nuclear reactions are discussed later. Essentially, the nuclei of two atoms can combine at very high temperatures, which releases tremendous amounts of energy in the form of heat and light.

## Reactions

Hydrogen, although having one outermost electron, does not fit into the alkali metals or any other group. It forms compounds analogous to those of the alkali metals, but such hydrogen compounds are much less alkaline (or more acidic), much less ionic, and more volatile. Sodium chloride, common salt, is neutral and clearly ionic; hydrogen chloride is a non-ionic gas under normal conditions and is a strong acid. The hydrogen analogue of sodium hydroxide is a volatile liquid (water, its most common compound) under normal situations—unlike the strongly alkaline and solid sodium hydroxide, water is slightly ionic and effectively neutral.
Hydrogen is a non-metal, forming a diatomic gas which results from the sharing of the single electrons of hydrogen atoms. It can achieve a stable ionic structure (no electrons!) by losing an electron or by gaining an electron and achieving the completed shell configuration of helium. The hydrogen molecule is best described as sharing the two electrons between two hydrogen atoms. This structure is highly stable and has little inclination to form bonds between other hydrogen molecules; hydrogen is a gas down to some of the lowest temperatures known. It is also the lightest of gases, weighing less even than helium.
| ${\hbox{H}}_{2(g)}+{\hbox{F}}_{2(g)}\to 2{\hbox{HF}}_{(g)}+heat$ | Hydrogen readily shares its electron with a strongly electronegative element, like any halogen, oxygen, or sulfur. The combination with fluorine is particularly violent and possible down to very low temperatures. Light is enough to force combustion between hydrogen and chlorine, and a spark is enough to cause combustion between hydrogen and oxygen. |
| $2{\hbox{H}}_{2(g)}+{\hbox{O}}_{2(g)}\to 2{\hbox{H}}_{2}{\hbox{O}}_{(g)}+heat$ | Due to the Hindenburg disaster of 1936, helium has long replaced hydrogen in lighter-than-air aircraft. The reaction responsible for that disaster was simply the combustion of hydrogen and oxygen. (Note that at the temperatures associated with such a combustion, water is in the gaseous state) |
| ${\hbox{H}}_{2(g)}+2{\hbox{Na}}_{(s)}\to 2{\hbox{NaH}}_{(s)}$ ${\hbox{NaH}}_{(s)}+{\hbox{H}}_{2}{\hbox{O}}_{(l)}\to {\hbox{NaOH}}_{(aq)}+{\hbox{H}}_{2(g)}$ | Hydrogen can act somewhat like a halogen, forming hydrides with some metals. Most of these react violently with water to form hydrogen gas and the metal hydroxide. Hydrogen compounds with non-metals are typically among the most volatile substances of those elements. |
| $2{\hbox{Zn}}_{(s)}+2{\hbox{HCl}}_{(aq)}\to 2{\hbox{ZnCl}}_{(aq)}+{\hbox{H}}_{2(g)}$ ${\hbox{M}}+2{\hbox{H}}^{+}\to {\hbox{M}}^{2+}+{\hbox{H}}_{2(g)}$ (general net ionic equation) | Under pressure, in aqueous solutions, or in non-solid acids, hydrogen is a good reducing agent. Strong acids attack most metals. This example shows hydrochloric acid added to zinc. In the atmospheres of gas giant planets (Jupiter, Saturn, Uranus, and Neptune) gaseous hydrogen under great pressure reduces nitrogen to ammonia, carbon compounds to methane and other hydrocarbons, and oxides to water. |

## Compounds

Almost all carbon compounds contain hydrogen, and vice versa, but more substances containing hydrogen (without carbon) exist than do compounds of carbon (without hydrogen). Hydrogen forms bonds with most non-metals, including oxygen, nitrogen, and carbon. Although a hydrogen atom can bond with only one other element, and then only in a single bond, hydrogen allows very long chains of carbon atoms to form. Most of the hydrogen compounds with carbon alone are combustible gases or volatile liquids or waxy solids that can be vaporized and burned to produce water, carbon dioxide, and much heat. Natural gas, gasoline (a mixture of liquid hydrocarbons), and waxes as found in candles make suitable fuels. With such other elements as oxygen, nitrogen, sulfur, and in some cases metals, hydrogen allows the formation of substances necessary for life, including carboxylic acids, sugars, proteins, nucleic acids, haemoglobin, and chlorophyll.
Such complex compounds are ordinarily discussed in Organic Chemistry, a study associated more obviously with carbon.

## Forms

Hydrogen has three isotopes. All hydrogen atoms contain exactly one proton in the nucleus, but there can be zero, one, or two neutrons. 99.98% of all hydrogen atoms naturally found on Earth have no neutrons. This is called protium, or $^1$H. It is stable, along with deuterium, or $^2$H. Deuterium has one neutron. It behaves exactly like regular hydrogen, but it weighs twice as much. Thus, "heavy water" is D$_2$O, where D is deuterium. Tritium, or $^3$H, has two neutrons. It is unstable (radioactive) and decays into helium.
If hydrogen loses an electron, it becomes H$^+$, simply a bare proton. In an aqueous solution, H$^+$ forms hydrogen bonds with a surrounding water molecule to create hydronium H$_3$O$^+$. As the hydronium concentration of a solution increases, so does its acidity.
Although such a reaction is uncommon, hydrogen can gain an electron to become an H$^-$ ion.