Why don't electrons crash into the nuclei they “orbit”?

Question

I'm having trouble understanding the simple "planetary" model of the atom that I'm being taught in my basic chemistry course.

In particular,

1. I can't see how a negatively charged electron can stay in "orbit" around a positively charged nucleus. Even if the electron actually orbits the nucleus, wouldn't that orbit eventually decay?
2. I can't reconcile the rapidly moving electrons required by the planetary model with the way atoms are described as forming bonds. If electrons are zooming around in orbits, how do they suddenly "stop" to form bonds.

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I understand that certain aspects of quantum mechanics were created to address these problems, and that there are other models of atoms. My question here is whether the planetary model itself addresses these concerns in some way (that I'm missing) and whether I'm right to be uncomfortable with it.

Answer 1

You are right, the planetary model of the atom does not make sense when one considers the electromagnetic forces involved. The electron in an orbit is accelerating continuously and would thus radiate away its energy and fall into the nucleus.

One of the reasons for "inventing" quantum mechanics was exactly this conundrum.

The Bohr model was proposed to solve this, by stipulating that the orbits were closed and quantized and no energy could be lost while the electron was in orbit, thus creating the stability of the atom necessary to form solids and liquids. It also explained the lines observed in the spectra from excited atoms as transitions between orbits.

If you study further into physics you will learn about Quantum Mechanics and the axioms and postulates that form the equations whose solutions give exact numbers for what was a first guess at a model of the atom.

Quantum mechanics is accepted as the underlying level of all physical forces at the microscopic level, and sometimes it can be seen macroscopically, as with superconductivity, for example. Macroscopic forces are limiting cases of the real forces which reign microsopically.

Answer 2

This is just my 2 cents:

Electrons have so little mass they are at times a wave, their mass is so negligible don't think of them as planets, rather representations of energy.

Sometimes electrons do fall into the nuclei this is nuclear decay.

Answer 3

Briefly,

1. The Bohr--planetary model doesn't really address these issues.

Bohr, a genius, just asserted that the phenomena at the atomic level were a combination of stationarity while being in an orbit, and discrete quantum jumps between the orbits. It was a postulate that yielded some agreement with experiment and was very helpful for the future development of quantum mechanics solely because it got people to think about stationarity and discreteness.

2 It is totally useless for discussing chemical bonds. You are quite right to be uncomfortable with it.

3 It would be stretching a point, but you could see the Quantum Mechanics of Heisenberg and Schroedinger as the only way to salvage the planetary model of Bohr, by finally coming up with an explanation for the stationarity of an electron's state around (but no longer considered as « orbiting ») the nucleus and an explanation for discrete jumps as a response to perturbations from outside. But this required seeing the electron more as a wave and hence not having any definite location along the orbit.