Exception?

Chemistry Level 2

What is the full electron configuration of Lanthanum?

(A) : $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{2} 4p^{6} 4d^{10} 5s^{2} 5p^{6} 5d^{2} 6s^{1}$

(B) : $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{2} 4p^{6} 4d^{10} 5s^{2} 5p^{6} 5d^{1} 6s^{2}$

(C) : $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{2} 4p^{6} 4d^{10} 4f^{1} 5s^{2} 5p^{6} 6s^{2}$

(D) : $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 3d^{10} 4s^{2} 4p^{6} 4d^{10} 5s^{2} 5p^{6} 5d^{3}$

1 solution

Ahaan Rungta
Jan 30, 2014

We know that the shortened electron configuration of Lanthanum is $\text{[Xe]} 5d^1 6s^2$ , so the answer must end it $5d^1 6s^2$ , so we are done.

Its pretty easy if you are given the shortened electron configuration, although this configuration is actually an exception to Madelung's Rule, hence the name of the problem. If you were to determine the shortened electron configuration by hand, using Aufbau's principle you wouldn't get the actual configuration of Lanthanum. This is because in higher energy shells, electrons are sometimes more stable in a higher energy shell than the one they should be in using the standard rules due to the stability properties of half filled and filled subshells. In higher energy shells the difference in energies between successive shells is much lower making these exceptions more prevalent.

Cole Coupland - 7 years, 4 months ago

Exception?

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