Lecture 12: Aufbau Principle and Hund's Rule

Aufbau Principle - Building up of electrons

No more than 2 electrons can occupy an orbital or no 2 electrons in an atom can have all 4 quantum numbers the same. In other words- No 2 electrons in an atom can occupy all Y quantum # at the same time. Every electron in an atom has a unique set of the 4 quantum #. At least 1 quantum # should be different for 2 electrons.


Order of Increasing Energy (n+l)

1S2,2S2,2P6,3S2,3P6,4S2,3d10,4P61S^{2},2S^{2},2P^{6},3S^{2},3P^{6},4S^{2},3d^{10},4P^{6}

Electrons fill up orbitals by the increasing order of energy


Hund's Rule of Maximum Multiplicity

  • Electrons occupy degenerate orbitals singly if possible
  • Electrons will occupy a set of degenerate orbitals with parallel spin as far as possible
  • but other combinations are also possible but they are of higher energy of excited states

2P3^{3} Px Py Pz

\uparrow \uparrow \uparrow \rightarrow Obeying the Hund's Rule

\uparrow \downarrow \uparrow \rightarrow Against Hund's Rule (Possible but for higher energy)


Extra Stability for Half-filled and Filled Shells.

  • Be has a completely filled (S2) Configuration \rightarrow stable \rightarrow more energy is needed to remove an electron
  • B has one electron in 2P \rightarrow removed easily \rightarrow low IE
  • N has 2p3^{3} and is half-filled \rightarrow stable \rightarrow high IP
  • O has 2p4^{4} and the 4th^{th} electron is in Px were there is already an electron \rightarrow it is easy to remove \rightarrow low IP


Sum of power - # Periodic Table


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