![]() In addition, the atomic radius increases down a group, just as it does in the s and p blocks. Next comes the seventh period, where the actinides have three subshells (7 s, 6 d, and 5 f) that are so similar in energy that their electron configurations are even more unpredictable.Īs we saw in the s-block and p-block elements, the size of neutral atoms of the d-block elements gradually decreases from left to right across a row, due to an increase in the effective nuclear charge ( Z eff) with increasing atomic number. After the 4 f subshell is filled, the 5 d subshell is populated, producing the third row of the transition metals. From this point through element 71, added electrons enter the 4 f subshell, giving rise to the 14 elements known as the lanthanides. Although La has a 6 s 25 d 1 valence electron configuration, the valence electron configuration of the next element-Ce-is 6 s 25 d 04 f 2. Further complications occur among the third-row transition metals, in which the 4 f, 5 d, and 6 s orbitals are extremely close in energy. For example, Nb and Tc, with atomic numbers 41 and 43, both have a half-filled 5 s subshell, with 5 s 14 d 4 and 5 s 14 d 6 valence electron configurations, respectively. In the second-row transition metals, electron–electron repulsions within the 4 d subshell cause additional irregularities in electron configurations that are not easily predicted. Table 23.1 Valence Electron Configurations of the First-Row Transition Metals Sc Because the ns and ( n − 1) d subshells in these elements are similar in energy, even relatively small effects are enough to produce apparently anomalous electron configurations. In Chapter 7 "The Periodic Table and Periodic Trends", we attributed these anomalies to the extra stability associated with half-filled subshells. Unexpectedly, however, chromium has a 4 s 13 d 5 electron configuration rather than the 4 s 23 d 4 configuration predicted by the aufbau principle, and copper is 4 s 13 d 10 rather than 4 s 23 d 9. With two important exceptions, the 3 d subshell is filled as expected based on the aufbau principle and Hund’s rule. As we go across the row from left to right, electrons are added to the 3 d subshell to neutralize the increase in the positive charge of the nucleus as the atomic number increases. The valence electron configurations of the first-row transition metals are given in Table 23.1 "Valence Electron Configurations of the First-Row Transition Metals". Electronic Structure and Reactivity of the Transition Metals ![]()
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