Question of the Week

Do electrons perpetually spin around atoms?

Tue, 24th Dec 2013

Part of the shows Hydrogen-powered Party Poppers, Why don't electrons fall into the atomic nucleus? and Why don't microwaves spark off themselves?

Question

jd asked:

Hi Chris,

 

always wondered how come an electron seems to "perpetually" spin around a nucleus? What forces are involved and how come friction doesn't play a part etc etc?

 

with thanks

 

john from greensborough, Victoria Australia

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Around 1913, the physicist Niels Bohr described a model of the atom where negative electrons, attracted by the positive charge of the nucleus, spin around in circular orbits.

This model is often drawn like a miniature solar system, with the nucleus in the center, and electrons following circular orbits in a flat plane. In reality:

The electrons completely surround the nucleus in three-dimensional shells,

These shells often take on more complex shapes, some of them more like the orbit of comets than planets

In a planetary system, it is unstable to have several planets in the same orbit. However, it is common for two electrons share the same "orbital" inside an atom.

In a planetary system, it is possible for a planet to lose a tiny amount of angular momentum (eg via friction with the solar wind), and spiral in slightly closer to the star. However, inside an atom this is forbidden by quantum rules which declare that only specific orbitals are stable, and that at most 2 electrons can share each orbital.

Gravity only attracts planets and their star. In an atom, the electrons repel each other.

While it is possible to measure the position and velocity of a planet with great accuracy, in the quantum world of an atom, it is not possible to precisely measure the position and velocity of an electron. It is only possible to state that there is a certain probability of finding an electron in a certain location.

Under some circumstances, it is possible to get a fraction of electrons to align with an external magnetic field, but normal atomic vibrations often mix up the directions once the external field is removed.


The idea of perpetual rotation is probably derived from textbook drawings of the Bohr model of the atom, rather than representing a physical reality.

"Friction" is a property which is apparent in large-scale phenomena consisting of trillions of atoms, but which not nearly so obvious on the quantum scale of individual electrons orbiting a nucleus in an isolated atom.

For more on this topic, see the thread at: http://www.thenakedscientists.com/forum/index.php?topic=49320.msg421854#msg421854 evan_au, Tue, 3rd Dec 2013

Do electrons perpetually spin around atoms? No.

The Bohr model has some value in inorganic chemistry but clearly doesn't represent the truth as an orbiting electron will lose energy in the form of electromagnetic radiation (photons).

The "orbital" solutions to the electron wave equations are probably named out of respect for Bohr, de Broglie, and others whose development of the orbiting electron concept yielded many useful insights, but this collection of symmetric clouds and dumbells doesn't involve actual movement of charged particles, and explains stereochemistry far better than the Bohr atom.    alancalverd, Wed, 4th Dec 2013

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