The Structure of Atoms

To understand spectral lines you have to understand some Atomic Physics.

• What makes an element?

  The number of protons in the nucleus of an atom is what defines an atom.

• For example: Hydrogen (H) atoms have 1 proton (p⁺).
  • H with p⁺ and 1 electron (e^-) is "neutral" hydrogen (¹H₁).
  • H with p⁺ and e⁺ and a neutron (n⁰) is a "heavy" isotope of Hydrogen called deuterium (²H₁).
  • If a p⁺ is added to Hydrogen, we then have a different element - Helium (⁴He₂).

  

  Q. How many neutrons in ²³⁸U₉₂?

  Looks like a total of 238 nucleons, and 92 p⁺. So, it must have 238 - 92 = 146 n⁰.

• What does this have to do with light and spectral lines?

  Lots of clever experiments around the turn of the century demonstrated that :

  1. Light can be thought of as a stream of "quanta" called photons. Each photon carries an energy E = h (h is "Planck's constant", is the frequency).

  2. Atoms had a crazy structure in which only certain orbits were allowed for the electrons - that word again, the orbits are "quantized".

  

• "g" = "ground" state = lowest energy configuration

• "1" = 1st excited state = higher energy configuration

• "2" = 2nd excited state = higher energy yet

• etc. right on up to where the electron is no longer bound to the atom.

Atoms and Spectra

One of Nature's most basic rules:

Systems naturally seek their lowest available energy state - or - logs roll downhill

• So, left alone, hydrogen atoms tend to be in the ground state.

Now, let's bombard an H atom with photons. What happens? Most of the photons go zipping right past without interacting with the H atom. But! photons with just the RIGHT energy get absorbed by the atom

Right means that the energy of the photon corresponds to the energy level difference between "allowed" orbits in the H atom. Absorbed means that the energy of the photon will now be gone and the atom will be in a higher energy state.

• A photon with frequency will be absorbed by an atom if the energy of the photon corresponds to an energy level difference between allowed states in the atom

• What happens next? Remember, Nature seeks the lowest available energy level so the e^- bumped to an excited orbit will drop back to the ground state.

• Another Law of Nature, Conservation of Energy , states that the energy difference between the excited state and ground state must appear somewhere when the atom makes the transition. "Somewhere" is as a photon with the energy of the original one.

  

• Below is a schematic diagram of the allowed orbits in a Hydrogen atom. If you can answer the questions about it, you've got the idea.

  

  1. Which transition(s) correspond(s) to the absorption of a photon?

  2. Which transition corresponds to the highest energy photon emitted ?

  3. Which transition corresponds to the shortest wavelength photon emitted?

  4. Which transition corresponds to the highest frequency photon emitted?