State Bohr's first and second postulates. Use them to derive an expression for the radius of the $\displaystyle \mathrm{n}^{\text {th }}$ orbit in a hydrogen atom.

Statements of Bohr's first and second Postulates \(\displaystyle \frac{1}{2}+\frac{1}{2}\) Derivation of expression for radius of \(\displaystyle \mathrm{n}^{\text {th }}\) orbit $\displaystyle 2$ Electrostatic force between revolving electron and nucleus provides requisite centripetal force \[\frac{m v_{\mathrm{n}}^{2}}{r_{\mathrm{n}}}=\frac{1}{4 \pi \varepsilon_{0}} \frac{e^{2}}{r_{\mathrm{n}}^{2}} \]