Subshells electron-filling order

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

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. 

The n + t rule governs the order of increasing energy of the electrons in the atom. Subshells are filled with electrons in the order of increasing + , with due regard for the limitations on the quantum numbers and for the Pauh exclusion principle. In each case, the last electron can be added to the configuration of the element before, using a procedure known as the build-up principle. (Section 4.4)... 

Aufbau ( building up ) Principle A guide for predicting the order in which electrons fill subshells and shells in atoms. 

The building-up order of subshells is obtained by following the arrow through this array of subshells. The 1 s-subshell is filled first, and electrons are added to subsequent subshells as traced by the arrow. 

FIGURE 7.23 The order in which atomic subshells are filled in a many-electron atom. Start with the 1 s orbital and move downward, following the direction of the arrows. Thus the order goes as follows 1 s < 2s < 2p < 3s < 3p < 4s < 3d <. . . . ... 

FIGURE 3.10 In this "building-up" version of the periodic table, the lightest elements are at the bottom. Electrons fill subshells from bottom to top in order of energy as the atomic number of the atom increases. The numbers across the top give the number of electrons in each subshell. The ground-state electron configurations of most elements are apparent from their positions in the table. Those that are known to differ from expectation are indicated explicitly. 

In order to elucidate the spectroscopic terms belonging to a given electronic configuration, the first step is to ascertain the electron configuration using the atom-building principles (a), (b), and the additional rules (c), (d). The number of Pauli-allowed combinations N s (number of microstates) should be also calculated so that the correctness of the terms obtained can be checked. For a subshell partly filled with electrons, N s is given by the expressions ... 

In a many-electron atom the subshells are filled in the order shown in Figure 7.21. For electrons of the same principal quantum number, their penetrating power, or proximity to the nucleus, decreases in the order s > p > d > f. This means that, for example, more energy is required to separate a 3s electron from a many-electron atom than is required to remove a 3p electron. 

An orbital diagram (shown as circles) is the notation used to show the number of electrons in each subshell. Each subshell is labeled with its subshell notation, s, p, d, or f. An orbital diagram also makes it easy to see the sequence of how subshells are filled. If you use small circles to stand for a subshell, then the orbital diagram can be used to find the orbital configuration of nearly every element. Figure 6.2 shows the order of an orbital filling sequence. 

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. 

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 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 ... 

---