Exceptions to Electron Configurations

Electron configurations are another way to represent quantum numbers.  Both electron configurations and quantum numbers indicate the location of electrons while in its ground state for a particular atom.  The periodic table is split into 4 blocks, designated as s, p, d, or f, each representing a different shape for of the orbitals.  This also corresponds to the azimuthal quantum number l (that’s the lowercase letter L).  On the periodic table, s corresponds to groups 1 and 2, p corresponds to groups 13 through 18, and d corresponds to groups 3 through 12.  The f orbitals are found at the bottom of the periodic table and are filled with the inner transition metals, also known as the lanthanide and actinide series or elements.

In the larger subshells of d and f (each with 10 and 14 orbitals, respectively) they sometimes do not fill their orbitals in the predicted manner.  For example, chromium, element 24.  One would expect to have an electron configuration of [Ar] 4s² 3d⁴.  However, the electron configuration for chromium is found to be [Ar] 4s¹ 3d⁵.  One electron from the outer s orbital is promoted to the d subshell.  It is able to do this because the 4s and 3d subshells are very close in energy.  This gives both the outermost s and d subshells a complete set of half-filled orbitals.  This is a more stable configuration because it reduces electron repulsion within orbitals as much as possible and reduces the overall energy.  It holds that other elements found in the same group as chromium behave the same way.

Furthermore, copper behaves similarly.  Cu has an expected electron configuration of [Ar] 4s² 3d⁹ but in reality demonstrates [Ar] 4s¹ 3d¹⁰.  One electron from the outermost s orbital is promoted to the d orbital to give it a complete subshell.  Again, the reason for this has to do with lower energy and greater stability.  The elements found beneath copper on the periodic table behave in a similar fashion.  Filled subshells are the most stable, followed by half-filled subshells and then lastly incomplete subshells.

1.      It has been hypothesized that elements in the f-block elements behave the same way.  Predict which elements you would expect to have exceptional electron configurations and explain why.

2.      Explain why silicon does not demonstrate an electron configuration of [Ne] 3s¹ 3p³.

3.      Identify the other transition metals besides chromium and copper that demonstrate exceptional electron configurations.