Explanation of element line emission spectra.

[howlingmadpanda]

http://jersey.uoregon.edu/elements/Elements.html I recently stumbled upon this link while looking for a good tool to use to help identify elements by spectra, but I noticed some things I didn't quite understand. Why is it that Thorium and Uranium have such a wide ranged emission spectrum while plutonium has a much smaller one. I read that the more electrons and higher shell configurations give more possibilities to emit light, especially if the electrons are in higher shells. Could somebody clarify?