Question #aa3cf
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Plenty, but at the general chemistry level, copper and chromium are the most common "exceptions" and the ones you should definitely know.
The exceptions listed in my textbook that give nonstandard configurations are:
#color(blue)("Cr")# (chromium),#color(blue)([Ar]4s^1 3d^5)# #color(blue)("Cu")# (copper),#color(blue)([Ar]4s^1 3d^10)# #"Nb"# (niobium),#[Kr]5s^1 4d^4# #color(blue)("Mo")# (molybdenum),#color(blue)([Kr]5s^1 4d^5)# #"Ru"# (ruthenium),#[Kr]5s^1 4d^7# #"Rh"# (rhodium),#[Kr]5s^1 4d^8# #"Pd"# (palladium),#[Kr]4d^10# #color(blue)("Ag")# (silver),#color(blue)([Kr]5s^1 4d^10)# #"La"# (lanthanum),#[Xe]6s^2 5d^1# #"Ce"# (cerium),#[Xe]6s^2 4f^1 5d^1# #"Gd"# (gadolinium),#[Xe]6s^2 4f^7 5d^1# #"Pt"# (platinum),#[Xe]6s^2 4f^14 5d^9# #color(blue)("Au")# (gold),#color(blue)([Xe]6s^2 4f^14 5d^10)# #"Ac"# (actinium),#[Rn]7s^2 6d^1# #"Th"# (thorium),#[Rn]7s^2 6d^2# #"Pa"# (protactinium),#[Rn]7s^2 5f^2 6d^1# #"U"# (uranium),#[Rn]7s^2 5f^3 6d^1# #"Np"# (neptunium),#[Rn]7s^2 5f^4 6d^1# #"Cm"# (curium),#[Rn]7s^2 5f^7 6d^1# #"Lr"# (lawrencium),#[Rn]7s^2 5f^14 6d^0 7p^1# #"Ds"# (dermstadtium),#[Rn]7s^1 5f^14 6d^9# #color(blue)("Rg")# (roentgenium),#color(blue)([Rn]7s^1 5f^14 6d^10)#
Obviously, you do not have to memorize all of these. Just a few of them, which I will go into below.
Many of these "exceptions" follow different principles that are seemingly conflicting, so not all of these are "half-filled
For many transition metals, the
Only a few exceptions, like yttrium (
Since copper and chromium both have one electron left until a filled
- By Hund's rule to maximize the total number of parallel electrons ("one at a time and double up" is how I was first taught it), demotion of a
#4s# electron down to a lower-energy#3d# orbital for chromium gives its more stable electron configuration by reducing the electron repulsion. - Due to energy arguments, demotion of a
#4s# electron down to a lower-energy#3d# orbital for copper gives its more stable electron configuration purely due to the difference in orbital potential energies (#"5.05 eV"# , or#"487.25 kJ/mol"# ). The total number of parallel electrons, however, remains the same, so Hund's rule isn't applicable here.
The elements that follow similar principles in their electron configurations (giving a resultant half-filled
#"Mo"# , but not#"W"# and not#"Sg"# !, each below#"Cr"# #"Ag"# ,#"Au"# , and#"Rg"# , each below#"Cu"#
Note that these are not easily explainable, so you do not have to know why these configurations yield half-filled
Any half-decent explanation should involve a complicated mixture of Hund's rule, energy differences between the
