Half filled exception

[Adrienne B 1J]

Hi,

my TA mentioned a half filled exception today for electron configuration but I'm not quite sure what is meant by this.

[004985802]

it may have been referring to the d block favoring being half or fully filled (meaning having d^5 or d^10) so if the atom has two electrons in the valence s orbital and 4 or 9 electrons in the d valence orbital, an electron will move from the s to the d orbital to have it half or fully filled

[Emily 1E]

Chromium is an example of the half filled exception. Instead of being Cr = [Ar] 4s^2 3d^4 as expected following the usual rules, it is 4s^1 3d^5. Because the 3d sub shell is more energetically stable if it has one electron in each of its 5 orbitals, that if it were to have only 4.

[Ani Boyadjian 1A]

This would occur in an atom with an electron configuration where its (n-1)d orbital contains 4 or 9 electrons. The atom will be more stable if it takes an electron from the (n)s^2 orbital making the (n-1)d orbital contain 5 electrons (half-full) or 10 electrons (full orbital) while (n)s orbital will contain 1 electron (ns^1).