What are the frequency and wave length of a photon emitted during a transition from the n=5 state to n=2 state in the hydrogen atom?

Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number?

The energies of the electron in the second and the third Bohr orbits of the hydrogen atoms are -5.42xx10^(19)J and -2.41xx10^(19)J respectively. Calculate the wavelength (in S.I. unit) of the emitted radiation, when the electron jumps from the third to the second orbit.

The spectral line at 434 nm in the Balmer series of the hydrogen spectrum corresponds to a transition from nth orbit. What is the value of n?