L2: Bohr's Model and Atomic Spectra

Problems with Rutherford’s Model

• Orbiting electrons were acceleration, but there is no emission of EMR

• Electrons are losing energy, but they are not spiralling towards the nucleus

Continuous Spectrum

• Heated solids, liquids, and dense gases

• Emit light with a continuous spectrum (contains all frequencies)

• There are no gaps in EMR due to the collisions between atoms

Bright Line Spectrum (Studied)

• Low pressure (rarefied) gases exposed to high voltages give off specific colours

• Emitted EMR is due to the excited atoms themselves

• Each element has a unique emission spectrum (classical theory could not explain why most frequencies were missing)

Dark Line Spectrum (Evidence)

• When white light passes through unexcited gas, there are dark lines in the spectrum

• Wavelengths absorbed matched wavelengths emitted

Bohr’s Energy Level Model

• The spectra lines corresponded to differences between quantized energy levels in the hydrogen atom

• Orbits in an atom are quantized

  • Each orbit corresponds to a particular energy level for the electron since the energy of an electron is quantized

  • Electrons can move between energy levels by emitting or absorbing energy equal to the difference between energy levels

• Only light with certain frequencies &

  wavelengths can be emitted or absorbed by an atom

• Energies of an electron within an atom are most often

  given relative to ionization

  • The zero point for energy is when electron is an infinite

    distance from nucleus

  • The negative sign means electron needs that much

    energy to be removed from atom (ie: to ionize atom)

Transition Energy

• Energy of the photon that an atom absorbs or emits

• Conservation of Energy (PP5)

  • ΔE_e^- = E_photon

  • E_f (energy in final orbit) - E_i (energy in initial orbit) = hf

• Positive E = Photon emitted

• Negative E = Photon absorbed

Evaluate Bohr’s Model

• Positive: Explained emission and absorption spectra for hydrogen (and other 1 electron atoms)

• Negative:

  • Could not explain the spectra for more complex atoms and why only certain orbits are allowed

  • Could not explain why orbiting electrons did not emit EMR and why some lines were brighter than others