Question

Why does in Bohr's model of atoms the orbits start from K and not from A?
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Answer 1

The names of the electron shells come from a fellow named Charles G. Barkla, a spectroscopist who studied the X-rays that are emitted by atoms when they are hit with high energy electrons. He noticed that atoms appeared to emit two types of X-rays. The two types of X-rays differed in energy and Barkla originally called the higher energy X-ray type A and the lower energy X-ray 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.
As it turns out, 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. Barkla won the 1917 Nobel Prize for Physics for this work.

Answer 2

The names were coined by Charles Glover Barkla. He was studying the x-rays emitted by atoms when they are hit with high energy electrons [1]. He noticed that there are two types of characteristic x-ray lines produced, which have different energies. He named the higher energy lines as A and lower one as B. However, it was changed to K and L after he realized that the energy of A need not be the highest energy possible so that characteristic x-rays of higher energies can be named J, I etc. when found. 

However, it later turned out that K has the highest energy possible. But the name stuck. 

Henry Moseley investigated the relation between characteristic x-rays and atomic number and found that the square root of characteristic x-ray energy is directly proportional to the atomic number. [2]

This result, when viewed in the light of the Bohr model of atoms, shows that the series of lines K, L etc correspond to electron shells. Thus, these names started being used for shells. 

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