When examined with a grating spectrograph,the discharge from helium gas has bright radiation at a relatively few discrete wavelengths; the discharge from nitrogen gas has radiation in bands with a very large number of discrete wavelengths.What is the reason for this difference in the spectra of these two gases?
A)The atoms of the helium gas have two electrons whereas the atoms of the nitrogen gas have many electrons; therefore,the nitrogen gas gives a more complicated spectrum.
B)The nitrogen gas has molecules that have rotational and vibrational energies that produce bands of closely spaced wavelengths in the spectrum.
C)The nitrogen gas discharge is hotter,which "smears out" the wavelengths of the radiation according to Planck's law for blackbody radiation.
D)The levels of the helium atoms can be obtained from the Bohr theory of atoms,which gives a few relatively discrete allowed energies,whereas nitrogen must be described by the band theory,which gives continuous bands of allowed energies.
E)None of these is correct.

Question

Quizplus · Accepted Answer

The bands of closely spaced wavelengths in the spectrum of nitrogen gas are caused by rotational and vibrational energies of its molecules. This is known as the rotational-vibrational spectrum. In contrast, the relatively few discrete wavelengths in the spectrum of helium gas are due to the fact that its atoms have only a few allowed energy levels. The other options are incorrect. The number of electrons in an atom does not necessarily determine the complexity of its spectrum, and the temperature of the discharge does not explain the difference between the spectra of helium and nitrogen. Finally, while the Bohr theory of the atom does give discrete energy levels, it is not applicable to molecules like nitrogen, which require the use of quantum mechanics and the band theory to explain their spectra.

When Examined with a Grating Spectrograph,the Discharge from Helium Gas