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Hydrogen Atoms
(mander.xyz)
A place for majestic STEMLORD peacocking, as well as memes about the realities of working in a lab.

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Just integrate phi squared over the sphere space, and with that set to .999, solve for r. Use something like Maple for the algebra if you're a wimp.
Assuming the probability distribution falls off like a gaussian, I'd guess something like 3.5 to 4 times as large. I dont remember if that specific probability density function is gaussian or not, but assuming a distribution falls off like a gaussian is usually a safe bet
it is not gaussian. for first electron, it is closer to poissonian