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Normally, the electron configuration of Te is known as:

$$\begin{aligned} {[Kr]} 5s^2 \ce{4d^10} 5p^4 \end{aligned}$$

Then, one day I was asked in a exam if this can be written also as:

$$\begin{aligned} {[Kr]} 5s^2 \ce{4d^10} 5p^3 6s^1\end{aligned}$$

I answered that it couldn't. But my answer turned out to be wrong.

If this is correct, why is it allowed to be written like this? And what general case can I learn when being asked such tricky questions?

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The second configuration allows Te to take advantage of the relative stability of half-full shells. Having the 5p and 6s orbitals both half-populated is more stable than having 4 electrons in the 5p orbital.

Similar behavior is what allows carbon to form 4 bonds. The $\ce{s^1p^3}$ configuration is more stable than the $\ce{s^2p^2}$.

Tellurium shows the behavior while oxygen does not as a result of the overlap between the 5p and 6s orbitals in the larger atoms. The same degree of overlap does not occur with the 2p and 3s orbitals.

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  • $\begingroup$ To add to this, consider the energy level diagram found here. You can see at the lower energy levels the s and p orbitals are well separated and the distinction between the 1, 2, and 3 energy levels are also large. Beyond that the various orbitals become quite close in energy, so lots of unexpected things occur. $\endgroup$ – Jason Patterson Oct 6 '14 at 16:50

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