A heavy atom has lots of
electrons surrounding the nucleus in various shells. To keep it simple I
didn't show them all. Really that electron that drifts in comes from one of
the other shells of the atom. The K-shell knock-out affects the innermost
electron, so its like having a hole in the bottom. This "hole" causes a
domino effect where the electrons above it cascade down to fill the hole.
But why the innermost electron? I would have guessed the outermost electron
would be the easiest to knock out.
An x-ray photon has a lot of energy in it, and only transitions of the inner
electrons release that much energy. Transitions of the outer electrons, which
can happen, might be in the infrared or visible part of the spectrum. For the
electron energies used in x-ray tubes, it turns out the inner electrons are
the most likely to be knocked out.
You said earlier that the K-shell spectrum
depends on the target element. Why?
What did Moseley actually do?
Back in 1913 when he was a graduate student he looked at the K-shell radiation
coming out from various elements from aluminum to gold. He found a connection
between the wavelength of the emitted K-shell x-ray and the element (atomic
number) that helped us understand atoms better.
How can you find the wavelength of an x-ray? That seems like a hard thing to
That's another story in itself. He used the (then) recently developed
technique called Bragg scattering, where you scatter x-rays off a crystal. He
was able to predict the existence of elements not yet observed, such as
technetium, promethium and rhenium. And even today, because of the uniqueness
of the fingerprints of each element, x-rays are used for chemical analysis as
it is very sensitive to impurities.