Exceptions to the Building-Up Principle or Aufbau Principle

University:
North Central College
Course:
CHEM 121 | General Chemistry I
Academic year:

2023
Views:

352

Pages:

1
Author:

do3ran7ut

Exception to the Building-Up Principle (Aufbau Principle) Chromium is an exception to how the Aufbau Principle predicts electron configurations. For example, the Aufbau principle predicts the following electron configuration for chromium: Cr: [Ar] 3d44s2 (Again, recall that one way we can write the configuration is in increasing subshell order, and so the 3d is of lower energy than the 4s subshell) However, the general rule is that for transition metals that have lost two or more electrons from their neutral atom state, we try to fill or half-fill the d-orbital first because it is of lower energy than the 4s subshell. So, in this case since we need one more electron to halffill the d-orbital (or get to 5 electrons), so we take one electron from the outermost shell (the 4s) subshell, and put it in the d-orbital to half-fill it, leaving one electron in the 4s subshell. So, the actual electron configuration for Chromium is as follows: Cr: [Ar] 3d54s1 Now, if we are talking about the Cr2+ ion we have lost 2 electrons, so the electron configuration for Cr2+ is as follows: Cr2+: [Ar] 3d4 So Cr2+ has four unpaired outermost electrons

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