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Absorption and emission

Valence Electrons can make transitions between the orbitals by absorbing and emitting a discreet amount of energy

  • The amount of energy absorbed and emitted must be exactly the energy difference between the two orbitals
  • The energy absorbed places the atom in an excited state
  • The exact amount of energy absorbed must then be radiated, as per the energy difference of orbitals

Emission Series

  • Since the emissions of light are of exactly the same energy as the energy difference between the electron orbitals, the light that’s emitted will be very specific
  • The emitted light from an electron dropping back down to the energy level n=1 (Lyman series) will be too high energy and will not be visible
  • The opposite is true for an electron dropping back down to an energy of n=3 (Paschen series)
  • Electrons dropping back down to energy level n=2 (Balmer series) will be visible Fraunhofer Lines
  • Since only certain electron excitations/emissions will be visible and statistically probable for each element, various lines for different elements are assigned letters and used for characterisation purposes

Sodium D line

  • A perfect example is the sodium D line
  • Since the valence electron occupies the 3s orbital, the most common excitation and emission will the path from the \(\ce{3p -> 3s}\)
  • This results in a significantly brighter emission at 589 nm than any other wavelength.


List of more character spectral lines

Designation Element Wavelength (nm)
y O2 898.765
Z O2 822.696
A O2 759.370
B O2 686.719
C 656.281
a O2 627.661
D1 Na 589.592
D2 Na 588.995
D3 or d He 587.5618
e Hg 546.073
E2 Fe 527.039
b1 Mg 518.362
b2 Mg 517.270
b3 Fe 516.891
b4 Mg 516.733
c Fe 495.761
F 486.134
d Fe 466.814
e Fe 438.355
G’ 434.047
G Fe 430.790
G Ca 430.774
h 410.175
H Ca+ 396.847
K Ca+ 393.366
L Fe 382.044
N Fe 358.121
P Ti+ 336.112
T Fe 302.108
t Ni 299.444

Hyperfine Spectrum

  • When observed at very high resolution, spectral lines split can split into two (sodium \(D_1\) and \(D_2\))
  • They are caused by an interaction of the atom’s nuclear magnetic dipole moment, due to the distribution of charge within the atom.
  • The distance of the orbitals can vary, so slightly that there is a marked difference in the energy absorbed and emitted.

Absorption and Emission

  • Since the excitation and emission processes happen in conjunction with each other, absorbance is proportional to the emission and an absorption spectra is ultimately an inverse emission spectra



  • Due to the specific nature of the electron configuration of each element, due to it’s valence, electronegativity, mass, etc. Emission spectra can be used as a fingerprint of a different elements