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. Unfortunately, there is no easy way to explain these deviations in the ideal order for each element. 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. Hence, Cr and Cu are called exceptional configuration. Since chromium had 4 electrons, which is one short of 5 electrons to get just hslf-filled. The d-orbital can occupy a maximum of 10 electrons. this happens with Nb and Ag which are underneath Cr and Cu respectively. Therefore we have (still incorrect) 1s 2 2s 2 2p 6 3s 2 3p 6 3d 9 4s 2. The electron configurations of chromium and copper seem to disagree with what is expected according to the Aufbau principle. Three d nine. Copper atoms are said to have a configuration of 3d 10 4s 1 as opposed to 3d 9 4s 2 as might have been expected from the general trend. In learning chemistry, chromium, and copper, the 3d elements of our environment reveal their general form of electron or electronic structure trends in the periodic table. 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. Correct Electron Configuration for Copper (Cu) 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#.. Reason. Video: Cu, Cu+, and Cu2+ Electron Configuration Notation. The completely filled orbitals and half filled orbitals are highly stable as in the case of copper and chromium respectively. There are two main exceptions to electron configuration: 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). Half filled subshells feature lower electron-electron repulsions in the orbitals, thereby increasing the stability. i. Similarly, completely filled subshells also increase the stability of the atom. Half-filled and completely filled subshells seem to be more stable. I have started studying chemistry and I was told to memorise the exceptional electronic configurations of Chromium and Copper. the reason for abnormality in the electronic configuration of Chromium and Copper is the maximum stability. The sequence of filling of electrons in 3 d subshell gets disturbed in chromium and copper and these elements possess exceptional configuration. Chromium is said to have a configuration of 3d 5 4s 1 as opposed to 3d 4 4s 2. 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