The first radius of helium ion from Bohr model of an atom is:

Bohr Radius =${\mathrm{R}}_0{\mathrm{n}}^2/\mathrm{z}$​

     where,     R₀​=0.529A˚

Now, first orbit of He⁺  n=1, z=2.

                     R= ${\mathrm{R}}_0/2$=0.2645 A˚

     ∴ The radius of first orbit  in He⁺ ion is 0.2645 A˚ (or)26.45pm