Inside of an atom snapped | Hacker News Reader

Inside of an atom snapped

stuff.co.nz

47 pointsjonbaer15 comments

Am I the only one very surprised at the scale? Millimeters? I would expect much, much smaller... am I reading this wrong?

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...

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

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)

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....

[2] http://www.ucolick.org/~krumholz/courses/spring10_ast230/not...

I couldn't dig up the original article, but I wouldn't be too surprised if the images were in fact millimetres in size.

The images are of electrons scattered from hydrogen atoms, directed onto a plate by an electromagnet. The scattering almost certainly has a lens effect, similar to shining a torch at a mirror ball. From the article:

the team fired two lasers at hydrogen atoms inside a chamber, kicking off electrons at speeds and directions that depended on their underlying wave functions.

A strong electric field inside the chamber guided the electrons to positions on a planar detector that depended on their initial velocities rather than on their initial positions.

I read the lead in the same way. I thought,"oh, haha, silly innumerate journalists, they must have meant tenths of nanometers, and they even got that wrong!" and I felt the superiority that comes from catching someone in a factual error.

However, it's the imaging mechanism that produced a correspondingly large picture of an atom of hydrogen.

So then I felt normal again, perhaps a little more ashamed than when I started reading the article.

See http://news.rice.edu/2008/06/26/physicists-create-millimeter...

I think this is a different, unrelated article.

In the experiment you linked to the researchers managed to force an electron into a huge orbit, on the scale of millimetres. In the OP's article they are imaging normal Hydrogen atoms.

It's very interesting, but unrelated as far as I can tell.

In semi-conductor devices, silicon atoms are considered to be 0.25 nanometres in diameter. I cant imagine the orbitals of a much smaller atom to be bigger.

This scale would have to be a size projected onto a detector. The scale of the source would then be determined by the details of the experiment.

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

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

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

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.)

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

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?

Hydrogen electrons 'orbit' in shells, not a uniform sphere

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