Table 30.3Electron Configurations of Elements Hydrogen Through Calcium
Element Number of electrons (Z) Ground state configurationH (^11) s^1
He (^21) s^2
Li (^31) s^22 s^1
Be 4 " 2 s^2
B 5 " 2 s^22 p^1
C 6 " 2 s^22 p
2
N 7 " 2 s^22 p^3
O 8 " 2 s^22 p
4
F 9 " 2 s^22 p^5
Ne 10 " 2 s^22 p^6
Na 11 " 2 s^22 p
6
3 s^1
Mg 12 " " " 3 s^2
Al 13 " " " 3 s^23 p
1
Si 14 " " " 3 s^23 p^2
P 15 " " " 3 s^23 p
3
S 16 " " " 3 s^23 p^4
Cl 17 " " " 3 s^23 p^5
Ar 18 " " " 3 s^23 p^6
K 19 " " " 3 s^23 p^64 s^1
Ca 20 " " " " " 4 s^2
Examining the above table, you can see that as the number of electrons in an atom increases from 1 in hydrogen to 2 in helium and so on, the
lowest-energy shell gets filled first—that is, then= 1shell fills first, and then then= 2shell begins to fill. Within a shell, the subshells fill starting
with the lowestl, or with thessubshell, then thep, and so on, usually until all subshells are filled. The first exception to this occurs for potassium,
where the 4 ssubshell begins to fill before any electrons go into the 3 dsubshell. The next exception is not shown inTable 30.3; it occurs for
rubidium, where the 5 ssubshell starts to fill before the 4 dsubshell. The reason for these exceptions is thatl= 0electrons have probability
clouds that penetrate closer to the nucleus and, thus, are more tightly bound (lower in energy).
Figure 30.62shows the periodic table of the elements, through element 118. Of special interest are elements in the main groups, namely, those in
the columns numbered 1, 2, 13, 14, 15, 16, 17, and 18.
CHAPTER 30 | ATOMIC PHYSICS 1101