
Inside of an atom snapped - jonbaer
http://www.stuff.co.nz/science/8721115/Inside-of-an-atom-snapped/
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flebron
Am I the only one very surprised at the scale? Millimeters? I would expect
much, much smaller... am I reading this wrong?

~~~
ngoldbaum
While in this case, the microscope is apparently producing a magnified image
of a hydrogen atom's orbital structure, it is possible for an atom to have a
macrscopic size, although it normally only happens in deep space. It turns out
that the mean radius if an electron relative to the proton in a hydrogen atom
scales with the square of the principle quantum number, n [1].

Since the energy scales with one over the square of n, high-n states all have
energies close to zero. A very tenuously bound electron, with a very large n,
can reach macroscopic size scales. While the ground state of hydrogen is the
only truly stable state, in tenuous environments with low collision rates,
atoms can get "stuck" in excited states that only have very rare radiative
transitions available. These sorts of very excited atoms are called Rydberg
atoms [2].

[1]
[http://info.phys.unm.edu/~ideutsch/Classes/Phys531F11/Proble...](http://info.phys.unm.edu/~ideutsch/Classes/Phys531F11/ProblemSets/Phys531_PS01.pdf)

[2] <http://en.wikipedia.org/wiki/Rydberg_atom>

~~~
steve19
Theoretically, how wide could an atomic radius get in deep space?

Or does that mean that there is an infinitesimally small chance a electron
could be found very far away from the nucleus because there is nothing else
for it to interact with in empty deep space?

(The physics problem you linked to is ___far_ __beyond my understanding)

~~~
ngoldbaum
I'm honestly not sure how large atoms can be, since there are an arbitrarily
large number of high-n states in the limit where the electron is tenuously
bound to the proton. I know that states with n ~ 100 are commonly observed
(see Figure 3 of [1]) in Galactic and extragalactic star forming regions. This
is because ionizing radiation creates a population of relatively recently
recombined atoms where the electron happened to arrive in a high-n state.

It's not that the electron has an infinitesimally small chance to be observed
far away from the nucleus - it's quite likely actually if it lives in a high-n
state - it's that collisions rates are so low that the system is out of
thermodynamic equilibrium. Section III of these notes [2] describe the physics
of Rydberg aroms and the radio recombination lines we observe from them quite
nicely.

[1]
[http://iopscience.iop.org/0004-637X/549/2/979/pdf/52126.web....](http://iopscience.iop.org/0004-637X/549/2/979/pdf/52126.web.pdf)

[2]
[http://www.ucolick.org/~krumholz/courses/spring10_ast230/not...](http://www.ucolick.org/~krumholz/courses/spring10_ast230/notes9.pdf)

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ColinWright
More details about the imaging techniques given in the article submitted
yesterday:

<https://news.ycombinator.com/item?id=5767401>

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chris_l
So shouldn't the electron density be a sphere? It has this strange ring around
outside...

~~~
colanderman
The 2D projection of a spherical shell isn't a uniform disc – it does indeed
look more or less like a ring. (Consider the rough analogy of a swinging
pendulum – it spends most of its time near the edges of its swing.)

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Thrymr
An orbital is not a spherical shell.

Edit: These are probably higher energy states. e.g., the 6s orbital:
<http://en.wikipedia.org/wiki/File:HydrogenOrbitalsN6L0M0.png>

edit 2: Previous comment was still not correct. More plots of electron density
for different energy states: <http://cronodon.com/Atomic/AtomTech4.html>

~~~
colanderman
From the page you linked: "Notice that the electron is most likely to be found
at one Bohr radius from the centre, in approximate agreement with the
classical atomic model." That's approximately a spherical shell.

Why would these be such high-energy states?

