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If electrons can't be confined to the nucleus, why does K-electron capture occur?

If the uncertainty principle forbids confinement of an electron to the nucleus, then why do the isotopes potassium-40, beryllium-7, (as well as a few others) decay by electron capture in which a proton in-fact captures an electron? Kraushaar, Wilson, and Bainbridge (Phys. Rev. 90:610 (1953)) have determined that a change in the half-period of electron capture can be effected by chemical composition (circa 0.013%).
Michael Prescott (eximer@netcom.com)

Michael, There isn't a dilemma. The calculation shows only that the electron can't be restricted to a space as small as an atomic nucleus. The process you describe doesn't involve confinement of an electron. It involves a transformation of an electron (and a proton) into a neutron. If the electron retained its identity as an electron, after the transformation- then there would be a problem.

The capture of s electrons by a proton occurs within an unstable, neutron-rich nucleus. Usually it's the 1s electrons that are captured. When a vacancy opens up in the 1s orbital, an electron from a higher shell drops into its place, dumping energy as X-rays. The X-rays are experimental evidence that electron capture by the nucleus is taking place. Luis Walter Alvarez was the first to demonstrate nuclear decay via K-shell capture in 1937 (just a year out of grad school).

I haven't seen that paper, but I imagine that anything that affects the 1s electrons could affect the electron capture rate. Since the 1s electrons can be perturbed slightly by valence shell events, it's reasonable to expect that you'd have a rare instance of a nuclear reaction that can be influenced by a chemical reaction. I'd also expect the coupling between the two to be pretty weak, because the 1s electrons are really not very sensitive to chemistry.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! If electrons can't be confined to the nucleus, why does K-electron capture occur?

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