The radius of the second Bohr orbit for hydrogen atom is

(Planck's constant, $\left(\mathrm{h}\right)=6.6262\times {10}^{-34}\mathrm{Js};$ mass of electron $=9.1091\times {10}^{-31}\mathrm{kg};$ charge of electron $=1.60210\times {10}^{-19}\mathrm{C};$ permittivity of vacuum, $\left({\mathrm{\in }}_0\right)=8.854185\times {10}^{-12}{\mathrm{kg}}^{-1}{\mathrm{m}}^{-3}{\mathrm{A}}^2$)

Radius of the Bohr's orbit is calculated by the following formula ${\mathrm{r}}_n=0.529\times \frac{{\mathrm{n}}^2}{\mathrm{z}}\mathrm{Å}$

Radius is directly proportional to square of n^th orbit and inversely proportional to atomic number. This formula works for single electron species

here ${\mathrm{r}}_{\mathrm{n}}=$radius of the nth orbit

$\mathrm{Z}=1 \mathrm{and} \mathrm{n}=$orbit or shell number

${\mathrm{r}}_2=0.529\times \frac{{\left(\mathrm{2}\right)}^2}{1} Å$

$=2.12 \mathrm{Å}$