Hydrogen has only one electron, yet it exhibits multiple lines in a spectral series, why is this?
As the electrons fall from higher levels to lower levels, they release photons. Different "falls" create different colors of light. A larger transition releases higher energy (short wavelength) light, while smaller transitions release lower energies (longer wavelength).
The visible wavelengths are caused a by single electron making the different transitions shown below. There are even more transitions that release invisible wavelengths.
Wavelength Transition Color (nm) --------------------------------------------- 383.5384 9 -> 2 Ultra Violet 388.9049 8 -> 2 Ultra Violet 397.0072 7 -> 2 Ultra Violet 410.174 6 -> 2 Violet 434.047 5 -> 2 Violet 486.133 4 -> 2 Bluegreen (cyan) 656.272 3 -> 2 Red 656.2852 3 -> 2 Red
*Values taken from Hyperphysics: Hydrogen Energies and Spectrum
(Why are there two different 3-> 2 transitions? See here: Hydrogen Fine Structure )
Though a hydrogen atom has only one electron, it contains a large number of shells, so when this single electron jumps from one shell to another, a photon is emitted, and the energy difference of the shells causes different wavelengths to be released... hence, mono-electronic hydrogen has many spectral lines.
Although hydrogen has only one electron, it contains many energy levels. When its electron jumps from higher energy level to a lower one, it releases a photon. Those photons cause different colours of light of different wavelengths due to the different levels. Those photons appear as lines. For this reason, though hydrogen has only one electron, more than one emission line is observed in its spectrum.
This arises due to the electron making transitions between the various energy levels in an atom . The electrons fall from a higher energy state to a lower energy state or vice versa emitting or absorbing wavelengths of different magnitude which fall directly on the spectrum of hydrogen thereby releasing colours of various wavelengths and creating a continuous spectrum .