Ch500Ch5LN3 - Chem 400 Chapter 5 Notes Part 3 Orbital...

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Chem 400 Chapter 5 Notes Part 3 Orbital Energy Diagram Exceptions You learned the above order for filling electrons in orbitals. But there are exceptions! 19 in fact! Most of the exceptions result from the following empirical fact: half-filled sublevels or subshells are more stable and filled sublevels are also more stable. As a result, sometimes electrons actually fill in a higher sublevel (violating the Aufbau Principle) in order to half-fill or fill it. This happens particularly for d sublevels which are very close in energy to the s sublevel right below it. An s electron often jumps to a higher d sublevel to half-fill the d sublevel (or to fill the d sublevel).
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The d sublevel thus gains stability, which compensates for the extra energy required to move the electron to the higher energy level. For example, in copper and chromium, a 4s electron jumps to the 3d subshell in order to half-fill it for Cr and to fill it for Cu. And the 4s orbital is no longer filled, but it is now half-filled, so it still has extra stability. It's actually more complicated, as there are several factors involved: electron-electron repulsions, energy required to promote or excite an electron, etc. In higher level chemistry classes, you would actually be able to prove that the atom becomes more stable even though the orbital diagram violates the rules for ground state diagrams. The common exceptions for you to know are chromium and molybdenum,
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Ch500Ch5LN3 - Chem 400 Chapter 5 Notes Part 3 Orbital...

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