Triunquadunpentennbihexpenttripentoctennseptenntribitrioctquadhexbihexquadtritriocttribiseptennpentnilbioctoctquadunennseptunhexenntriennenntriseptpentunnilpentoctbinilennseptquadennquadquadpentennbitrinilseptoctunhexquadnilhexbiocthexbiniloctennennocthexbioctniltriquadoctbipenttriquadbiununseptnilhexseptennium has the atomic number of pi (without the decimal point), or 31415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679.
It is impossible for this element to exist, for three reasons. First, it is physically impossible, since the number above is greater than 325. Second, there isn't enough matter in the observable universe to make an atom. This element's atomic number is around 3.1E100. If you took every last bit of matter and energy in the observable universe and turned it into protons, you would have less than a billionth of the protons needed to make a nucleus of this element - to say nothing of all those neutrons needed as well. Third, if an atom of this substance were magically generated, it would collapse into a supermassive black hole. Whether heavy-baryon matter, polyquark matter, or unconfined quark matter (maybe even unconfined top quark matter); it is reasonably safe to say that whatever's in a supermassive black hole won't be an atomic nucleus.