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Aufbau Principle Indicate which of the given electronic configurations are true ground state configurations for the respective atoms and which are false. (Take care! There are some exceptions to the simple rules.) false Si true F true Ra false 0 1s²2s²2p63d²3s² 1s²2s²2p5 [Xe]4f145d106s26p67s2 → 1s²2s²2d4 true K → 1s²2s²2p63s²3p64s²3d¹ The Aufbau rule can be used, most of the time, to predict which subshells will contain electrons, however, the actual order in which the subshells are shown here is not necessarily that suggested by the rule, but rather one which places the valence electrons last in order of relative energy for the atom concerned. In other words, don't worry too much about the order in which they are listed. Besides Cr and Cu, some other elements which have exceptional ground state configurations are: Nb, Mo, Ru, Rh, Pd, Ag, Pt, and Au. Their ground state configurations can be inferred by analogy with what happens in the cases of Cu and Cr. See Zumdahl, "Chemical Principles" 8th ed. Sec 12.13, p475 "The Aufbau Principle and the Periodic Table".