Electron Arrangement

• Aufbau principle: Electrons in an atom must fill the energy subshells in order from lowest to highest
• Pauli exclusion principle: There can only be a maximum of 2 electrons per orbital
• Hund's rule: Electrons cannot be paired until all orbitals in a sublevel has one electron
• Exceptions!

  There are few special cases/exceptions to electron configurations such as

  Copper, Cu: $[Ar]4s^1 3d^{10}$

  Chromium, Cr: $[Ar]4s^1 3d^5$