Absorption and Emission Spectra

Absorption and Emission Spectra

1. Energy Levels

   • An electron in a hydrogen atom can occupy any one of the fixed energy levels
   • These different energy levels are the same for all hydrogen atoms.
   • In the ground state, the electrons CLOSEST to the nucleus have the lowest energy.
   • The difference in the energy levels DECREASES as the electron moves away from the nucleus

2. Absorption Spectra

   • An absorption specctra seen on earth is the VISIBLE SPECTRUM OF LIGHT, with black lines corresponding to the absorptions of energy by the electrons
   • These spectra are seen from earth when atoms in the chromosphere around stars absorb light
   • Electrons absorb a 'photon'
     • Excited electrons move up to a higher energy level- they are PREMOTED. This is what produces the lines on the Absorption spectrum
       • The EM radiation absorbed by each hydrogen atom has a definite frequency, related to the difference in energy levels by AE=hv

3. Emission Spectra

   • Black background with coloured lines on it
     • These correspond to the emissions of energy by the electrons
   • An emission spectrum is seen when a chemical burns with a coloured flame
   • Electrons first absorb a 'photon'
     • Excited electrons move up to a higher energy level- they are premoted
       • Electrons then drop back to lower energy levels. This is what produces the lines on emission spectra
         • The E.R emitted by each H atom has a definite frequency related to the difference in energy levels by AE=hv
           • Since AE is different for each transition, the frequency is different for each transition, and so is the colour of the lines.

4. Similarities

   • Both line spectra
   • Lines in the same position for each element
   • Lines become closer at higher frequencies
   • Set of lines representing transitions to a particular level

5. Differences

   • Bright lines on black background OR black lines on coloured background