Final answer:
The presence of only one emission line in the gas bulb usually indicates that the element inside is hydrogen, as it has a single electron, whose transitions would generate a specific emission line.
Step-by-step explanation:
If a gas bulb shows only one emission line, it suggests the presence of an element with a very simple electron structure, typically hydrogen. Hydrogen is known to have only one electron, which when excited can emit light of a specific wavelength upon returning to its ground state. This single emission line is characteristic of hydrogen, as more complex elements tend to have multiple electrons and thus can produce multiple emission lines.
Helium, on the other hand, has two electrons but they share the same energy level or shell. According to the Pauli exclusion principle, no two electrons can have the same set of four quantum numbers, so while helium atoms do indeed have two electrons, their emissions would still show more complex spectra than a single line due to their interactions.
Therefore, the presence of a single emission line in the gas bulb most likely indicates that hydrogen is the element trapped inside, particularly since hydrogen's sole electron can transition between energy levels and give off one predominant spectral line in a certain condition.