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'''Hydrogen''' is the most abundant of all the [[element]]s,<ref>Hydrogen makes up roughly 75% of the matter in the [[universe]].</ref> and has the [[chemical symbol]] H and an [[atomic number]] of 1. Despite this abundance, naturally occurring elemental hydrogen is extremely rare on [[Earth]].

'''Hydrogen''' is the most abundant of all the [[element]]s,<ref>Hydrogen makes up roughly 75% of the matter in the [[universe]].</ref> and has the [[chemical symbol]] H and an [[atomic number]] of 1. Despite this abundance, naturally occurring elemental hydrogen is extremely rare on [[Earth]].

==Composition of the atom==

==Isotopes==

Hydrogen [[atom]]s normally consist of a single [[proton]] and [[electron]], with no [[neutron]]. Other [[isotopes]] are formed as the atom gains neutrons: [[deuterium]] (one neutron) and [[tritium]] (two neutrons). [[Tritium]] is [[radioactive]], with a [[half-life]] of approximately 12 years.  

The most common form of hydrogen is protium, although this name is rarely used and it is generally known simply as hydrogen.  Protium [[atom]]s normally consist of a single [[proton]] orbited by a single [[electron]]. Two other [[isotopes]] are known to exist: [[deuterium]], which has one neutron and an atomic weight of 2, and [[tritium]] with two neutrons and an atomic weight of 3. [[Tritium]] is [[radioactive]], with a [[half-life]] of 12.32 years. Naturally occurring tritium is extremely rare on Earth but has a number of valuable applications, such as light-emitting inserts for optical instruments and emergency equipment. As a result it is extremely expensive.  

==Compounds==

==Compounds==

Hydrogen can be found in a vast majority of [[organic]] compounds, as well as some [[inorganic]] compounds.<ref>For example, [[hydrocarbon]]s and metal hydrides, respectively</ref> It is an important component of [[water]] and most [[acid]]s.  

Hydrogen can be found in the vast majority of [[organic]] compounds, as well as many [[inorganic]] compounds.<ref>For example, [[hydrocarbon]]s and metal hydrides, respectively</ref> It is an important component of [[water]] and most [[acid]]s.  

==Fuel for vehicles==

==Fuel for vehicles==