explain why transition metals exhibit variable Oxidation State
Answers
The transition elements have outer electrons that occupy a very large shell.
This is a table that shows each element's outermost electron shell configuration.
The cyan colored elements are all transition metals (although the lanthanide and actinide series are also technically transition metals). The transition metals are all metals and therefore have a tendency to lose electrons, but because these electrons in the highest energy level are actually on a lower principal energy number than the ones before them (ie the first row of transition metals fills the 3d shell, and the 4s shell had already been filled), this makes it relatively easy to lose some or all of their electrons to land in stable states.
The d shell has a lot of stable states, and some of those states are actually more stable than the 4s subshell, hence the anomalies in the table above (in the fourth column in the transition metals you can see that the configuration is 3d5, and that the fifth column is also 3d5). According to Hund's rule, electrons first fill up shells with parallel spins before filling with the antiparallel spins. Since the d shell can hold 10 electrons, having 5 electrons is energetically stable because all five have parallel spins. This state is actually more stable than the 4s subshell and the electron configuration of Chromium shows that, rather than having an outer configuration of 3d4 as expected, Chromium takes an electron away from the 4s subshell and into the 3d subshell so that the d shell and the 4s shell are half filled with electrons with the same spin.
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