What is the value of a in the fine structure formula of alkali metal atomic spectrum So what is its calculation formula?

What is the value of a in the fine structure formula of alkali metal atomic spectrum So what is its calculation formula?

A is called the first Bohr radius. The orbital radius is also quantized, and its size can only be 1 (square of 1), 4 (square of 2) and 9 (square of 3)