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.
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Author: Fred Senese firstname.lastname@example.org