Atomic Spectra

• atoms can receive external energy via heat, electricity, and/or EM radiation (light/photons)
• when an electron absorbs a photon with energy equal to the difference between its current energy level and another, it enters an excited state and transitions to the higher energy level ¹
  • if the energy is too high, the electron can escape the atom and the atom becomes ionized
• excited electrons are unstable want to return to the most stable, e.g. the lowest possible, energy level, called the ground state
  • ground state — lower energy, stable
  • excited state — higher energy, unstable
• when an electron transitions from a higher energy level to a lower one, it emits a photon
• the energy/frequency of the emitted photon is determined by the difference in energy between the two levels the electron transitioned between
• each element has unique discrete energy levels where its electrons can exist
  • due to having different numbers of protons and electrons
• because of each elements’ unique energy levels, the light emitted by atoms of each element will have a distinct combination of frequencies
• this unique signature of frequencies is the atomic spectra of an element
• by analyzing the atomic spectra of some body [%, like a planet or star, we can determine its elemental composition
  • this is called spectral analysis
• molecules also have unique sets of energy levels, and therefor unique spectra

Energy levels

• atoms have an infinite number of energy levels, but a finite amount of energy is required for an electron to escape its atom
• an energy level diagram helps predict the photon energies by an atom

Two types of spectra

Absorption spectra

• light from another source (a star) passing through cooler gas (like a planet’s atmosphere) ²
• the cooler atoms absorb the light, but when the electrons transition down and emit their photons, the emitted photons are scattered in random directions
• the light that was travelling from the source to us/Earth, will have fewer of the specific frequencies absorbed/emitted

Emission spectra

• light directly emitted from excited atoms
• only the specific photon frequencies from electrons transitioning from higher to lower energy levels

Tools

• spectroscope — uses a diffraction grating to analyze the spectrum of a light source