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Why are the shells named K, L, M, ... instead of A, B, C...?

atomic number*
cathode ray*
principal quantum number*
x-ray spectra*

A few years ago my sister, who is an accomplished seamstress, was asked to mend a 19th century dress. She was delighted by the many ingenious compromises between economy and extravagance in the dress's design. But the 7" wide tucks of extra fabric she found in the shoulder and side seams puzzled her at first, since in those times material was valuable and never wasted. She realized that the dress was meant to last a lifetime, and the extra cloth was added to allow the dress to grow with its wearer over the years. Apparently, the dress's owner hadn't expanded in accordance with 19th century expectations.

The electron shell designations were started at K rather than A for similar reasons. Like the extra seam allowance on the dress, the letters A-J never had to be used.

In the early twentieth century, scientists were intensely interested in X-rays, which had just been discovered. Especially fascinating was the fact that elements emit X-rays when bombarded with streams of fast-moving electrons (as in J. J. Thomson's cathode ray tube). This discovery is the basis for medical and dental X-ray machines.

When high-energy electrons hit a sample of an element, they ionize its atoms in a rather unusual way. An inner shell electron is lost in the initial impact, and when it is recaptured, a great deal of energy is released in the form of X-rays.

The X-rays emitted seemed to be of two types. One type was able to penetrate sheets of metal of a certain thickness; the other wasn't. The spectroscopist who discovered this (Charles G. Barkla) named the more penetrating type A and the less penetrating type B, initially. But he worried that even more penetrating types of X-ray radiation would be discovered for other elements, so he renamed them K and L to leave room for more penetrating types A through J. These more penetrating radiations were never observed.

Barkla's work was continued by Moseley and others, and eventually it became the basis for an experimental procedure for determining the atomic numbers of the elements. For his contribution, Barkla received the 1917 Nobel prize in Physics.

Today we know that Barkla's K radiation is produced when electrons knocked out of the n=1 shell are recaptured. L radiation is produced when electrons knocked out of the n=2 shell are recaptured; since the n=2 shell has higher energy than the n=1 shell, less energy is released and L radiation is weaker (less able to penetrate metals) than K radiation.

Further information

The Nobel Prize in Physics 1917 (The Nobel Foundation)
The presentation speech for Charles Barkla's Nobel Prize, awarded "for his discovery of the characteristic Röntgen radiation of the elements". The site includes a portrait and a brief biography.
http://www.nobel.se/physics/laureates/1917/index.html (02/10/00)

X-Ray Spectra (John Parks, Diamond Bar High School)
A description of Charles Barkla's groundbreaking work connecting X-ray spectra with atomic weight, including his rationale for the KLMN... notation for X-ray series that eventually was adopted for electron shell designations. Part of John Park's ChemTeam site.
http://dbhs.wvusd.k12.ca.us/AtomicStructure/AtNum-XrayThread.html (02/10/00)

X-Rays (Physics2000, Colorado State University)
A gentle introduction to X-ray physics, including a discussion of X-rays in medical imaging, and general background on electromagnetic radiation and wave-particle duality. Part of Colorado State's Physics 2000 project.
http://www.colorado.edu/physics/2000/xray/index.html (02/13/00)

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Why are the shells named K, L, M, ... instead of A, B, C...?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/electrons/faq/why-KLMNOP.shtml