Electron Capture - What is It?

Atoms can be differentiated into two parts: the central nucleus--an inner core of positively charged protons plus uncharged neutrons--and an outer encasement of negatively charged electrons traveling in their various orbitals within their respective shells. Ordinary chemical processes involve the making or breaking of bonds between the outermost, or "valence," electrons of two or more atoms. The physics process called "electron capture" is the penetration by an inner shell electron into the nucleus of a neutron-rich atom. Absorption by one of the protons replaces it with a neutron. At the same time, a neutrino and an X-ray photon are emitted.

Elemental identity is determined solely by the number of protons in an atom's nucleus. Hydrogen has one proton; helium has two; chlorine has seventeen--there is a total of more than 100 elements in all. While the number of neutrons in a nucleus has no bearing upon the identity of an element, it does affect an element's atomic properties, most notably its nuclear stability. A number of varieties of the same element result when differing number of neutrons are found with the same number of protons in a nucleus--each of which is called an "isotope." When an electron capture occurs, it is not a mere change in isotope that occurs, but a change in element identity, since the number of protons in the nucleus has been lowered.

Since electron capture most often involves the first electron shell, also called the "K-shell," the process is sometimes called K-capture. Electron capture is not the only way unstable elemental isotopes can decay. There is also the process called "beta decay." This process requires greater initiation energy than that necessary for electron capture. Elements with a weight less than the more stable isotopes usually decay by electron capture, whereas those heavier that the more stable isotopes most often undergo the beta decay process.

There is another, unrelated process that might be confused with the electron capture process described above. Organic compounds, such as large biomolecules, can be bombarded with electrons in a form of mass spectrometry. These molecules undergo a kind of electron capture to produce odd-electron fragments. This technique joins a limited number of other important methodologies applicable to macromolecules. Biomolecular electron capture dissociation was first described by R. A. Zubarev of the Cornell University's McLafferty group in the 1998 article, Electron Capture Dissociation of Multiply Charged Protein Cations - a Nonergodic Process.

References and Resources

IUPAC Gold Book: Electron Capture

Georgia State University: Electron Capture

Purdue University: Radioactive Decay: Beta Decay: Electron Capture

Boston University School of Medicine: Electron Capture Dissociation