The band spectra (characteristic of molecular species) is due to emission of radiation from which state of matter?

If in nature there may not be an element for which the principal quantum number $n > 4,$ then the total possible number of elements will be

The colour of the positive column in a gas discharge tube depends on

When the potential of the second excited state of an atom is $108.8 \ V$,the recoil energy of the atom is ...... Joules.

The wavelength of the emission line spectrum and the absorption line spectrum of a substance are related as:

In a Franck-Hertz experiment,an electron of energy $5.6 \ eV$ passes through mercury vapour and emerges with an energy $0.7 \ eV$. The minimum wavelength of photons emitted by mercury atoms is close to ............. $nm$.