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I know that there are 3 isotopes of Hydrogen. But according to this website it is showing 7 http://periodictable.com/Properties/A/KnownIsotopes.htmlWebsite

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    $\begingroup$ As you can see by your link, heavier isotopes have decay time measured in $10^{-13}$ fractions of nanosecond. They are of theoretical interest for nuclear physics, but from chemists point of view can be ignored completely. $\endgroup$ – permeakra Feb 25 '15 at 16:53
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    $\begingroup$ Look at the data there. The half-life of 3H is 12 years. The half-life of 4H is 1.39×10-13 nanoseconds. Does that count as existence for you? $\endgroup$ – John Dvorak Feb 25 '15 at 16:53
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Yes, there are seven known isotopes of hydrogen, though only two ($\ce{^1H}$ and $\ce{^2H}$) are stable with respect to nuclear decay, and only three ($\ce{^1H}$, $\ce{^2H}$ and $\ce{^3H}$) exist/can be made in enough quantities to be relevant outside of nuclear physics. All other hydrogen isotopes have extremely small half-lives. The next most stable hydrogen isotope after $\ce{^3H}$ ($t_{1/2}=12.3\ yr$) seems to be $\ce{^5H}$ ($t_{1/2} \sim 10^{-21}\ s$). Given that even the fastest chemical reactions happen in timespans of the order of $10^{-15}\ s$, we can safely discard any influence of hydrogen isotopes heavier than $\ce{^3H}$ in chemistry.

Out of the currently 115 elements which have been confirmed to exist, there are approximately 254 isotopes which are considered to be indefinitely stable, or have half-lives so enormous (over trillions of times greater than the age of the Universe, 13.8 billion years) that no value could yet be determined. Theoretically, only 90 of those 254 isotopes are expected to be truly indefinitely stable (proton decay aside), but there are great difficulties in setting up experiments capable of measuring such rare decay events.

On top of the previous 254, another 34 isotopes have half-lives which have actually been measured in laboratory, but are still large enough (>50 million years) so that a small fraction of the isotope could conceivably have survived since the creation of the Earth 4.6 billion years ago, and may be of interest in geological timescales. There are also isotopes with smaller half-lives, but which are continuously replenished by the decay of heavier atoms, so they can still be found on Earth today. In all, about 339 different isotopes can be found naturally on our planet.

However, as many as 3100-3300 different isotopes of the first 118 elements are claimed to have been detected in laboratories, most of them with very small half-lives (seconds or less). Nobody knows for sure how many elements and how many isotopes can exist, though the number is finite.

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    $\begingroup$ Would a combination of other singly-positively-charged particles with an electron (positronium, muonium, bottomonium, etc.) be considered an isotope of hydrogen? Their chemistry should mimic that of hydrogen. $\endgroup$ – iad22agp Feb 25 '15 at 18:13
  • $\begingroup$ @iad22agp Good question. According to IUPAC, isotope is by definition a term applicable strictly to differing combinations of protons and neutrons. Anything not made solely of protons, neutrons and electrons is not an atom, but rather an exotic atom, of which there are several types depending on what gets replaced. $\endgroup$ – Nicolau Saker Neto Feb 25 '15 at 18:37

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