Electron Shells and Atomic Spectra

Electron Shells and Atomic Spectra

1. Electrons in Atoms are in Fixed Energy Levels

   • Elecrons movearound the nucleus in shells (energy levels)
   • Shells can only hold so many electrons
   • Ypu need to be able to describe the electron structure of the first 36 atoms in the Periodic Table.
   • The 3rd and 4th shells overlap, so even though the 3rd shellisn't yet full, the next two electrond go in the 4th shell. That the first 20 elements tacken care of.
   • Then it's back to ginish off the 3rd shell, followed by the rest of the 4th shell.

2. Electrons Absorb or Release Energy in Fixed Amounts

   • Atoms in their ground state have all their electrons at their lowest possible energy levels.
   • If an atom's electrons take in energy from their surroundings they can move to higher energy levels, further from the nucleus. At higher energy levels, electrons are said to be excited.
   • Electrons can also release energy be dropping down to a lower energy level.
   • The energy levels all have certain fixed walues -- they're discrete. Electrons can jump from one energy level to another by absorbing or releasing a fixed amount of energy
   • Absorption Spectra -- Made up of Dark Lines
     • Energy is related to frequency. So when electromagnetic radiation is passed through a gaseous element, the electrons only absorb certain frequencies, corresponding to differences between the energy levels
     • That means the radiation passing through has certain frequencies missing. A spectrum of this radiation is called an atomic absorption spectrum.
     • The missing frequencies show up as dark bands on a coloured background.
   • Emission Spectra -- Made Up of Bright Lines
     • When electrons drop to lower energy levels, they give out certain amounts of energy.
       • This produces lines in the spectrum too -- but this time it's called an emission spectrum.
         • For any particular element, the frequencies in an emission spectrum are the sme as those missing in the absorption spectrum
     • Each element has a different electron arrangement, so the frequencies of radiation absorbed and emitted are different. This means the spectrum for each element is unique

3. Energy is Related to Frequency

   • When an electron movesto a higher of lower shell it absorbs or emits electomagneitc radiation with a certain frequency.
     • The amount of energy aborbed or emitted is related to the frequency of the radiation by this equation:

4. Spectra are Made up of sets of lines

   • You get lots of sets of lines in spectra -- each set represents electrons moving to or from a different energy level.
     • So, in an emission spectrum, you get one set of lines produced when electroms fall to the n = 1 level, and another set produced when they fall to the n = 2 level, and so on.
   • Spectra often seem to make as much sense as bar codes.
     • But the emission spectrum of hydrogen is fairly simple becaues hydrogen only has one electron that can move.
     • It has three important sets of lines:

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