Question

Calculate the number of unpaired electrons in the gaseous ion cr3+

Answer 1

Cr usually has 24 electrons, so Cr 3+ would have 21 electrons.

First, you write the electron configuration of Chromium

1s2

2s2 2p6

3s2 3p6 3d5

4s1

Note that Chromium does not follow the Aubau principle, since a half full or full "d" orbital is the most stable, when you are close to being half or full in the d orbital (such as nd4 o4 nd5) an electron from the n+1s orbital will tranfer to the d-orbital, which is why we have 4s1 and 3d5 instead of 4s2 3d4.

Now, Chromium +3 loses 3 electrons so its configuration is:

1s2

2s2 2p6

3s2 3p6 3d3

It first loses one from the 4s1 orbital and two from 3d5, leaving use with 3d3.

Now, looking at the valence electrons of Cr 3+ (the electrons in the outermost shell) we see that in the 3s and 3p orbital all the electrons are paired but in the 3d orbital there are only 3 electrons. Since the d orbital can hold 10 electrons, that means that it can hold 5 pairs of electrons. Since we only have 3 electrons, they are all upaired and thus the Cr +3 ion has 3 unpaired electrons.