Reason. Video: Cu, Cu+, and Cu2+ Electron Configuration Notation. I have started studying chemistry and I was told to memorise the exceptional electronic configurations of Chromium and Copper. Both of the configurations have the correct numbers of electrons in each orbital, it is just a matter of how the electronic configuration notation is written (here is an explanation why). the reason for abnormality in the electronic configuration of Chromium and Copper is the maximum stability. Chromium is said to have a configuration of 3d 5 4s 1 as opposed to 3d 4 4s 2. Similarly, completely filled subshells also increase the stability of the atom. There are two main exceptions to electron configuration: chromium and copper. Half-filled and completely filled subshells seem to be more stable. Half filled or completely filled orbitals are more stable than those with one electron less. The electron configurations of chromium and copper seem to disagree with what is expected according to the Aufbau principle. Correct Electron Configuration for Copper (Cu) The completely filled subshells s 2 , p 6 , d 1 0 , f 1 4 and The half filled s 1 , p 3 , d 5 , f 7 subshells have extra stability. The d-orbital can occupy a maximum of 10 electrons. Since chromium had 4 electrons, which is one short of 5 electrons to get just hslf-filled. In these cases, a completely full or half full d sub-level is more stable than a partially filled d sub-level, so an electron from the 4s orbital is excited and rises to a 3d orbital. The sequence of filling of electrons in 3 d subshell gets disturbed in chromium and copper and these elements possess exceptional configuration. this happens with Nb and Ag which are underneath Cr and Cu respectively. The probable (expected) electronic configuration of chromium is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 4 4s 2 or [Ar] 3d 4 4s 2. ii The probable (expected) electronic configuration of copper is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 9 4s 2 or [Ar] 3d 9 4s 2. The completely filled orbitals and half filled orbitals are highly stable as in the case of copper and chromium respectively. Three d nine. i. Hence, Cr and Cu are called exceptional configuration. Half filled subshells feature lower electron-electron repulsions in the orbitals, thereby increasing the stability. The electron configuration for chromium is NOT #1s^2 2s^2 2p^6 3s^2 3p^6 3d^4 4s^2#, but #color(blue)(1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1)#.. Interestingly enough, Tungsten is more stable with an electron arrangement of #[Xe]4f^14 5d^4 6s^2#.. Therefore we have (still incorrect) 1s 2 2s 2 2p 6 3s 2 3p 6 3d 9 4s 2. 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