Periodic table group anomalies

The electron shells of all the elements in Group 1, for instance, are filled, except for a single electron in an outermost s orbital. In fact, most of the elements in any column of the periodic table have the same number of electrons in their outermost orbitals, the orbitals involved in chemical reactions. Those orbitals are usually the same type orbital—5, p, d, or/, though there are a few exceptions. As mentioned in Chapter 4, vanadium (Z = 23) has an unexpected quirk in the arrangement of the electrons in its outer orbitals. Platinum (Z = 78) exhibits a similar anomaly, as do a few other elements. Most elements, however, play by the rules. This is why the elements in a group behave similarly. 

Like the other Period 2 elements, carbon is an anomaly in its group, and its bonding ability makes it an anomaly in the entire periodic table. 

The order of filling is even more irregular than that suggested by this sequence. There exist about twenty exceptions to the simple aufbau scheme proposed by Bohr. This is one genuine sense in which the periodic table has not been reduced to quantum mechanics or atomic theory. Nevertheless, some recent group theoretical work has explained the sequence of 2,8,8,18,18 etc. in the order of filling of electron shells but not the anomalies mentioned above. (Kibler and Odabasi, 1973 Novaro, 1973 Kibler, 1989). 

---