Question

find radius of fourth orbit of helium positive ion

Answer 1

The Bohr-atom radius of any atom is given by the formula

r = 0.529 * n^2 / Z Angstrom
where n = number of orbit and Z is the atomic number of the element.

So,
radius of the fourth orbit of helium positive ion is given by,
r = 0.529 * (4)^2 / 2
= 0.529 * 16/2
= 0.529 * 8
= 4.232 Angstrom
= 4.232 * 10^-10 metres.