Hello

See as we know radius of stationary state of bohr model is given by
Rn=n square× Armstrong
So, for helium it will be
Rn=4×0.5×10^-10
= 2.0×10^-10

Hello,

We know that,

For nth Bohr's orbit, the formula of radius is,

r = ( 0.529 n^2 ) / Z

So, for third excited state of He+ ion,

n = 4 and Z = 2

So, r = ( 0.529 x 4^2 ) / 2

So, r = 8.464 / 2

So, r = 4 Å

So, Radius of Orbit of the electron in the third excited state of He+ will be 4 Å

Best Wishes.