Subshells filling order

The subshell-filling order in Figure 3.7 gives the electronic configurations shown below. Valence-sheU electrons are in darker type, and the noble gas configurations are verified. 

The subshell filling order is obtained from Figure 3.7 or Figure 3.8. F 9 electrons s 2s 2p ... 

The order of subshell filling is s2s2p3s3p4s3d4p5s4d5p6s4f 5d6pls5f 6d. An s subshell can have a maximum of 2 electrons, a p subshell can have 6 electrons maximum, a d subshell can have 10, and an / subshell can have 14. 

The order in which the different subshells fill up as successive electrons are introduced into the atom is listed below. It should be remembered that in this hypothetical advancement from one space in the Periodic Table to the next, not only is an electron being added, but also an additional proton to the nucleus. 

The seventh added electron will occupy the last empty 2p orbital, and the eighth, ninth, and tenth electrons will pair up with electrons already in the 2p orbitals. The tenth electron fills the 2p subshell, thus completing the second shell. This filling order is illustrated in I Figure 3.6. 

Figure 3.7 The relative energies and electron-filling order for shells and subshells.

It is often convenient to represent the electronic configuration of an atom in a concise way. This is done by writing the subshells in the correct filling order and then indicating the number of electrons in each subshell by a superscript. 

The suhsheU fiUing order is obtained from Figure 3.7 or Figure 3.8. All subshells except the last one are filled to their capacities as follows s subshells—filled with 2, p subshells— filled with 6, d subshells— filled with 10, and/subshells—filled with 14. The last subshell has only enough electrons added to bring the total number of electrons to the desired value. 

A similar pattern exists for the entire periodic table (T Figure 9.26). Notice that, because of the filling order of orbitals, the periodic table can be divided into blocks representing the filling of particular subshells. 

Electrons are added one by one for successive elements, filling each subshell in order of increasing energy. The electronic configurations of the first 18 elements are shown in Table 3.5. 

Nearly all the exceptions to the predicted filling order (so-called anomalous configurations) involve either filled or half-filled subshells. Explaining these excep>-tions is not only rather complicated but also probably best done case by case. 

An effective way to determine the detailed electron configuration of any element is to use the periodic table to determine which subshell to fill next. Each s subshell holds a maximum of 2 electrons each p subshell holds a maximum of 6 electrons each d subshell holds a maximum of 10 electrons and each / subshell holds a maximum of 14 electrons (Table 17-5). These numbers match the numbers of elements in a given period in the various blocks. To get the electron configuration, start at hydrogen (atomic number = 1) and continue in order of atomic number, using the periodic table of Fig. 17-10. 

Electrons fill the orbitals in order of increasing energy, meaning that the lowest energy subshells are filled first. This is known as the aufbau principle. Of course, some subshells, such as the p subshell and the d subshell, have degenerate orbitals. 

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