CBSE Class 9 Answered

Charles G. Barkla, a spectroscopist studied the X-rays that are emitted by atoms when they are hit with high energy electrons gave the names to the electron shells. He observed that atoms appeared to emit two types of X-rays. These X-rays differed in energy . He named the higher energy X-ray as type A and the lower energy X-ray as type B. He later renamed these two types K and L since he realized that the highest energy X-rays produced in his experiments might not be the highest energy X-ray possible. He wanted to make certain that there was room to add more discoveries without ending up with an alphabetical list of X-rays whose energies were mixed up.

Therefore, it was decided that the K type X-ray is the highest energy X-ray an atom can emit. It is produced when an electron in the innermost shell is knocked free and then recaptured. This innermost shell is now called the K-shell, after the label used for the X-ray. He also won the 1917 Nobel Prize for Physics for this work.