= 4) is occupied before the 3d subshell (n + = 3 + 2 = 5). The rule then predicts the electron configuration , abbreviated where [Ar] denotes the configuration of argon, the preceding noble gas. However, the measured electron configuration of the copper atom is . By filling the 3d subshell, copper can be in a lower energy state. Exceptions in the f-block The valence d-subshell often "borrows" one electron (in the case of thorium two electrons) from the valence f-subshell. For example, in uranium 92U, according to the Madelung rule, the 5f subshell (n + = 5 + 3 = 8) is occupied before the 6d subshell (n + = 6 + 2 = 8). The rule then predicts the electron configuration where [Rn] denotes the configuration of radon, the preceding noble gas. However, the measured electron configuration of the uranium atom is . All these exceptions are not very relevant for chemistry, as the energy differences are quite small and the presence of a nearby atom can change the preferred configuration. The periodic table ignores them and follows idealised configurations. They occur as the result of interelectronic repulsion effects; when atoms are positively ionised, most of the anomalies vanish. The above exceptions are predicted to be the only ones until element 120, where the 8s shell is completed. Element 121, starting the g-block, should be an exception in which the expected 5g electron is transferred to 8p (similarly to lawrencium). After this, sources do not agree on the predicted configurations, but due to very strong relativistic effects there are not expected to be many more elements that show the expected configuration from Madelung's rule beyond 120. The general idea that after the two 8s elements, there come regions of chemical activity of 5g, followed by 6f, followed by 7d, and then 8p, does however mostly seem to hold true, except that relativity "splits" the 8p shell into a stabilized part (8p1/2, which acts like an extra covering shell together with 8s and is slowly drowned into the core across the 5g and 6f series) and a destabilized part (8p3/2, which has nearly the same energy as 9p1/2), and that the 8s shell gets replaced by the 9s shell as the covering s-shell for the 7d elements. History The aufbau principle in the new quantum theory The principle takes its name from German, , "building-up principle", rather than