What is the number of half-filled orbitals in an atom of chromium?

1 Answer
Truong-Son N.
Dec 31, 2016

Remember that chromium, like molybdenum (but not tungsten), has a half-filled #3d# subshell:

#[Ar] 3d^5 4s^1#

The short version on why it's not #3d^4 4s^2# is that maximizing parallel electron spins minimizes the energy of the electron configuration. More detail on that should be lightly researched by reading this answer here.

All core orbitals are doubly-occupied. Therefore, they are not considered in the list of half-filled orbitals.

Half-filled orbitals contain one electron, but there are five #3d# orbitals (recall that #l = 2#, so the number of #m_l# is #2l + 1 = 5#). The above valence electron configuration looks like this:

#ul(uarr color(white)(darr))#
#4s#

#ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))#
#underbrace(" "" "" "" "" "" "" "" "" "" "" "" "" "" "" ")#

#color(white)(aaaaaaaaaaaaa)# #3d#

So, now you should see that there are six unpaired electrons, meaning that you have six half-filled orbitals.

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