Wednesday, November 10, 2010

midterm review #12

Q: Explain all the exceptions to electron configurations that we have learned including the odd filling order with d and f orbitals and the exceptions among the transition metals. Why do they occur?

A: Chromium and its respective group is an exception that exists in the d block. With these elements, an electron moves from the s sublevel to the d sublevel in order to create a half filled d sublevel. This occurs because a half filled d sublevel adds significant stability due to the fact that each of the orbitals will only have one electron (b/c hund’s rule states that they will all gain one electron before they double up). The presence of only one electron in each orbital minimizes repulsions and increases stability.

Copper and its corresponding group is also an exception. The same thing that happens with chromium occurs within this group in the sense that one electron leaves the s sublevel and is added to the d sublevel. The key differences between the two exceptions is that in the copper group elements add to the d sublevel to gain a completely full sublevel as opposed to a half filled sublevel. The reason the electron moves is similar to the reason that electrons transfer in the Chromium group-to add stability. 

A third exception that occurs is between the s and d orbitals of higher energy levels. For example, the 4s orbital fills before the 3d orbital. This occurs because the vast difference in energy between an s orbital and a d orbital is greater than the difference between energy levels. Therefore, a 3d orbital has more energy than a 4s orbital. This occurs throughout the energy levels as you move past the second level. A good way to see how they fill is to write out the orbitals by energy level and then imagine that they fill diagonally.