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==Isotopes==

==Isotopes==

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 consist of a single [[proton]] orbited by a single [[electron]]. Two other hydrogen [[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, emergency equipment, and lubricants that do not break down break down because of high heat. As a result it is extremely expensive and very desirable especially to military purchasers.

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 consist of a single [[proton]] orbited by a single [[electron]]. Two other hydrogen [[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==

As free hydrogen does not exist on Earth, hydrogen is not actually a source of energy but rather a carrier of it: its value as a fuel is that it provides a clean and efficient way to use energy produced by some other source. Due to the Second Law of thermodynamics the energy used to produce free hydrogen must be at least equal to the energy contained within it, meaning that at present it is not a viable fuel for most applications. Potential solutions include the use of bacteria that produce hydrogen through photosynthesis, which would effectively allow solar energy to be stored in hydrogen and used as fuel.<ref>http://www.making-hydrogen.com/hydrogen-from-bacteria.html</ref>

As free hydrogen does not exist on Earth, hydrogen is not actually a source of energy but rather a carrier of it: its value as a fuel is that it provides a clean and efficient way to use energy produced by some other source. Due to the Second Law of thermodynamics the energy used to produce free hydrogen must be at least equal to the energy contained within it, meaning that at present it is not a viable fuel for most applications. Potential solutions include the use of bacteria that produce hydrogen through photosynthesis, which would effectively allow solar energy to be stored in hydrogen and used as fuel.<ref>http://www.making-hydrogen.com/hydrogen-from-bacteria.html</ref>

==Notes and references==

==Notes and references==

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