3.3.1 Writing out electronic configurations
In Section 3.2, we described Figure 21 as an energy-level diagram, which represented the build-up of electronic configurations as electrons were inserted into sub-shells of progressively increasing energy. However, Figure 21 has been designed for just one purpose: to generate the correct electronic configurations in our thought experiment in which, to quote Niels Bohr, 'the neutral atom is built up by the capture and binding of electrons to the nucleus, one by one'. What Figure 21 tells us, at any stage of the thought experiment, is which of the still unfilled sub-shells has the lowest energy. That sub-shell then receives the next electron.
Figure 21 is therefore designed to give the order of energies only for those sub-shells that, at any stage of the thought experiment, are candidates for the reception of the next electron. It does not necessarily give the correct order of energies for all of the sub-shells in any one particular atom. Consider lead, atomic number 82.
Use Figure 21 to write out the electronic configuration of the lead atom.
In the lead atom, the occupied sub-shells of highest energy are 6s2 and 6p2. The four electrons in these sub-shells are the ones that most influence the chemistry of lead. They are the outermost electrons, and the most easily removed, being furthest from the nucleus. But the sub-shell sequence from Figure 21 does not give this impression. Although 6p2 appears at the end, suggesting that these are outermost electrons, 6s2 does not. A more correct order of energies in any particular atom is obtained by grouping the sub-shells first in order of increasing value of n, and then, within each n value, in the order s, p, d and f.
Do this for the configuration of the lead atom.
1s2|2s22p6|3s23p63d10|4s24p64d104f14|5s25p65d10|6s26p2. For clarification, the individual shells have been separated by vertical lines.
The electronic configurations of the atoms within this course have been written in this style. One of its merits is that the outer electrons with the highest principal quantum numbers appear at the right-hand end. In this case, they show that the outer electronic configuration of lead is of the type s2p2. This is less apparent in the earlier configuration that was derived directly from Figure 21.