222 CHAPTER 5: The Structure of Atoms
(n2, 1) and therefore also the second energy level. Electrons 11 through 17 fill the 3s
subshell (n3, 0) and partially fill the 3psubshell (n3, 1).
SolutionElectron n m ms eConfiguration1, 2 1 0 0 ^12 1 s^2
3, 4 2 0 0 ^12 2 s^2
21 1 ^12
5–10 2 1 0 ^12 2 p^6
21 1 ^12
11, 12 3 0 0 ^12 3 s^2
31 1 ^12
13–17 3 1 0 ^12 3 p^5
31 1 ^12 or ^12 **The 3p orbital with only a single electron can be any one of the set, not necessarily
the one with m1.
You should now work Exercises 104 and 108.THE PERIODIC TABLE AND ELECTRON
CONFIGURATIONSIn this section, we view the periodic table(see inside front cover and Section 4-1) from a
modern, much more useful perspective—as a systematic representation of the electron
configurations of the elements. In the periodic table, elements are arranged in blocks based
on the kinds of atomic orbitals that are being filled (Figure 5-31). The periodic tables in
this text are divided into “A” and “B” groups. The A groups contain elements in which s
and porbitals are being filled. Elements within any particular A group have similar elec-
tron configurations and chemical properties, as we shall see in the next chapter. The B
groups are those in which there are one or two electrons in the sorbital of the outermost
occupied shell, and the dorbitals, one shell smaller, are being filled.
Lithium, sodium, and potassium, elements of the leftmost column of the periodic table
(Group IA), have a single electron in their outermost sorbital (ns^1 ). Beryllium and magne-
sium, of Group IIA, have two electrons in their outermost shell, ns^2 , while boron and
aluminum (Group IIIA) have three electrons in their outermost shell, ns^2 np^1. Similar obser-
vations can be made for other A group elements.
The electron configurations of the A group elements and the noble gases can be
predicted reliably from Figures 5-28 and 5-29. However, there are some more pronounced
irregularities in the B groups below the fourth period. In the heavier B group elements,
the higher energy subshells in different principal shells have energies that are very nearly
equal (Figure 5-29). It is easy for an electron to jump from one orbital to another of nearly
the same energy, even in a different set. This is because the orbital energies are perturbed
(change slightly) as the nuclear charge changes, and an extra electron is added in going
from one element to the next. This phenomenon gives rise to other irregularities that are
analogous to those of Cr and Cu, described earlier.5-18