3.3 Electronic configurations and the Periodic Table

Figure 21 has been designed for use in a particular thought experiment. The purpose of the thought experiment is to see how the electronic configuration of the atoms changes as one moves through the Periodic Table from beginning to end. We start with the hydrogen atom, which has one proton and one electron. Then we proceed through a series of stages in each of which we add one new proton to the nucleus, and one new electron to the clutch of surrounding electrons. At each stage, the filling order of Figure 21 tells us what sub-shells are occupied, and how many electrons those occupied sub-shells contain. In addition, the filling of successive sub-shells in this thought experiment generates the form of the Periodic Table shown in Figure 22, in which the winding arrowed pathway follows the filling order of Figure 21. The different blocks of elements span regions in which particular types of sub-shells are being filled up. With the typical elements, the sub-shell type is either s or p; in the case of the transition elements, with rows of ten, it is d; for the lanthanides and actinides, with rows of fourteen, it is f. Indeed, because of this connection, one can think of Figure 22 as a demonstration of how the filling order of Figure 21 can be deduced from the form of the Periodic Table.

Figure 22 The winding pathway shows how the order of sub-shell filling (Figure 21) generates the full Periodic Table. Elements in the same column usually have similar outer electronic configurations. Hydrogen has been juxtaposed with helium to indicate the filling of the 1s sub-shell

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