The de-Broglie wavelength of the electron in the ground state of the hydrogen atom is $...... \text{ } \mathring{A}$ (radius of the first orbit of hydrogen atom $= 0.53 \text{ } \mathring{A}$).

For principal quantum number $n = 3$,the possible values of orbital quantum number $l$ are

The ionisation energy of $Li^{2+}$ is equal to

Radius of the second Bohr orbit of a singly ionized helium atom is ......... $ \mathring A $

The de-Broglie wavelength of an electron moving in the $n^{th}$ Bohr orbit of radius $r$ is

Obtain an expression for the frequency of radiation emitted when a hydrogen atom de-excites from level $n$ to level $(n-1)$. For large $n$,show that this frequency equals the classical frequency of revolution of the electron in the orbit.