Radii of Different Orbit of Hydrogen Like Ion

r the radius at which the electron revolves.

r = Ze²/4π ε₀v²


For hydrogen, z =1 and we get r1 as 53 picometre ( 1pm = 10^-12 m) or 0.053 nm. This length is called the Bohr radius and is a convenient unit for measuring lengths in atomic physics. It is denoted by me as the symbol a₀ (In HC Verma a different symbol is given. I am using this symbol as a convenience).

In terms of Bohr radius the second allowed radius is 4 a₀ and third is 9 a₀ and so on. In general nth orbit of hydrogen atom is n²a₀.