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However there are numerous exceptions; for example the lightest exception is chromium, which would be predicted to have the configuration 1s 2 2s 2 2p 6 3s 2 3p 6 3d 4 4s 2, written as [Ar] 3d 4 4s 2, but whose actual configuration given in the table below is [Ar] 3d 5 4s 1.
Grayed out electron numbers indicate subshells filled to their maximum. Bracketed noble gas symbols on the left represent inner configurations that are the same in each period. Written out, these are: He, 2, helium : 1s 2 Ne, 10, neon : 1s 2 2s 2 2p 6 Ar, 18, argon : 1s 2 2s 2 2p 6 3s 2 3p 6 Kr, 36, krypton : 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 ...
Ruthenium red, [(NH 3) 5 Ru-O-Ru(NH 3) 4-O-Ru(NH 3) 5] 6+, is a biological stain used to stain polyanionic molecules such as pectin and nucleic acids for light microscopy and electron microscopy. [66] The beta-decaying isotope 106 of ruthenium is used in radiotherapy of eye tumors, mainly malignant melanomas of the uvea. [67]
Electron configurations of the chemical elements ... 2s: 2p: 3 Li 1 - 4 Be 2 - 5 B 2 1 6 C 2 2 7 N 2 3 8 O 2 4 9 F 2 5 10 Ne 2 6 ... Ru 1 7 - 45 Rh 1 8 - 46 Pd-10 ...
Bohr's original configurations would seem strange to a present-day chemist: sulfur was given as 2.4.4.6 instead of 1s 2 2s 2 2p 6 3s 2 3p 4 (2.8.6). Bohr used 4 and 6 following Alfred Werner's 1893 paper. In fact, the chemists accepted the concept of atoms long before the physicists. Langmuir began his paper referenced above by saying,
Element Group Electron configuration electron configuration (P8000) Term symbol note 1, 2, S, P, 1 H hydrogen: 1 1s 1: 1s¹ : 2 S 1/2: S: 2 He helium: 18 1s 2: 1s² : 1 S 0: S: 3 Li lithium
Atomic number (Z): 44: Group: group 8: Period: period 5: Block d-block Electron configuration [] 4d 7 5sElectrons per shell: 2, 8, 18, 15, 1: Physical properties ...
The experimentally determined ground-state electron configurations of the elements differ from the configurations predicted by the Madelung rule in twenty instances, but the Madelung-predicted configurations are always at least close to the ground state. The last two elements shown, elements 119 and 120, have not yet been synthesized.