The ratio of the radius of the second orbit of hydrogen to the third orbit of $Li^{+2}$ is .... ($H$ atomic number = $1, Li$ atomic number = $3$)

Time taken for an electron to complete one revolution in the Bohr orbit of hydrogen atom is

What is the angular momentum of an electron in the orbit of an $H$ atom where the energy of the electron is $-3.4 \ eV$?

The value of the energy for the first excited state of hydrogen atom will be $............. \ eV$.

State whether the following statements are true or false:
$(i)$ The $Bohr$ model cannot explain how atoms form molecules.
$(ii)$ The $Bohr$ model is accepted as the planetary model.
$(iii)$ Electrons in an atom do not absorb or emit energy while moving in a stationary orbit.
$(iv)$ The spectral lines of the $Brackett$ series are found in the ultraviolet region.

The work function of a metal is $4.2 \, eV$. If radiation of $2000 \, \mathring{A}$ falls on the metal,then the kinetic energy of the fastest photoelectron is: