Electron Configuration A method we use to keep track of how electrons are arranged in an atom. It helps us to explain why atoms react the way they do. H 1s 1 Energy Level Sub-level # of electrons in sub-level The arrangement of the electrons may be represented in one of three ways. This is how hydrogen would be shown using spectroscopic notation Electron configuration may also be shown with box diagrams.

Electron Configuration A box is used to represent each orbital. An arrow is used to represent each electron. H 1s 1 These may also be called orbital box diagrams or orbital filling diagrams. Circles may also be used instead of boxes. Helium has 2 electrons: He1s 2

Electron Configuration Here are some more: Li1s 2 2s 1 Be 1s 2 2s 2 B 1s 2 2s 2 2p 1 C 1s 2 2s 2 2p 2 N1s 2 2s 2 2p 3 O 1s 2 2s 2 2p 4 F1s 2 2s 2 2p 5 Ne 1s 2 2s 2 2p 6 This arrangement (filled s & p in the outer energy level) is called a stable octet.

Spectroscopy How do we know that there are e- subshells in an atom? Actually, the same way we first found out about energy levels - Spectroscopy Many different forms of spectroscopy are used to determine charateristics and structural features for atoms and molecules UV/Vis Emission Spectroscopy Mass Spectroscopy Nuclear Magnetic Resonance Spectroscopy AA spectroscopy (UV) IR Spectroscopy (Bond types/functional groups) XPS – X-ray Photoelectron Spectroscopy (PES)

XPS uses X-rays generated from Al or Mg atoms that are focused by a small quartz crystal onto a sample of an element or compound. The energy is absorbed by electrons in different shells (or subshells) and they jump out of the atom. The KE of an electron is measured by a detector and is subtracted from the energy of the X-ray photon. This value represents the amount of energy that was necessary to remove that particular electron. Energy values and peak heights are plotted (Relative # of e - vs. Energy) For reasons I have yet to figure out, energy values are plotted backward (highest energy on the left, lowest on the right). These values can be compared to determine # of electrons in a shell, or compared with known spectra to determine the identity of an element.