The radius of 2ndBohr's orbit of hydrogen atom is:

Second Bohr orbit of hydrogen atom, i.e., n = 2.
Atomic number of hydrogen (z) = 1
by using 
$\begin{array}{l}\mathrm{r}=\frac{0.529{\mathrm{n}}^2}{\mathrm{Z}}=\frac{0.529\times (2{)}^2}{1}\\ =2.116{\mathrm{A}}^{\circ }=0.2116\mathrm{nm}\end{array}$