Radius of the first Bohr orbit of hydrogen atom is 0.05nm. Calculate the radius of the third orbit.

The radius of the $n$-th Bohr orbit is given by: $r_n = n^{2} r_1$ For the third orbit ($n = 3$): $r_3 = 3^{2} \times 0\cdot05\text{ nm} = 9 \times 0\cdot05\text{ nm} = 0\cdot45\text{ nm}$ Final answer: 0.45 nm