Energy levels and spectra

Bohr’s model of an atom

  • Atoms consist of a positive nucleus surrounded by electrons
  • The electrons can only exist in specific energy levels; there no levels in between
  • When a photon at the correct frequency is absorbed by the atom an electron jumps from a lower EL to a higher EL
  • A photon is emitted when an electron moves from a higher EL to a lower EL
  • An electron can move between any of the levels

Energy levels

Level 1 is the ground state. This occurs when the electron is closest to the nucleus.

Level ∞ is the ionisation state, when the electron is removed from the atom and the atom is ionised.

Photon absorption and emission

When an atom absorbs a proton of frequency (f), the difference in energy levels of the excited electron is given by:

ΔE = E2 – E1 = hf

Note: this is in joules!!!

Energy levels and spectra

Atoms can be excited by:

  • Heating
  • Bombardment by fast moving particles
  • Absorption of photons

When atoms are excited, their electrons remain in higher energy shells.

When they return to the ground state, they emit photons with similar energy levels.

Emission and absorption spectra

There are two types:

  • Emission spectra are the spectrum of light that is emitted by a light source
  • Absorption spectra are the spectrum of light that is absorbed by the material
Emission spectra

Line emissions. If gaseous atoms are excited by fast-moving particles, then light of specific frequencies is emitted.

Band emissions. It is similar to line emission except the spectra due to the excitation of molecules.

Continuous emissions. It occurs when light is emitted from a hot incandescent such as a hot white metal.




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