Untitled - Kelly Walsh High School

114 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
Maximum number of electrons for s-subshells = 2, p-subshells = 6, d-subshells = 10,
f-subshells = 14.
Let’s use cobalt, Co, to illustrate how to use Figure 7-2. Cobalt has 27 electrons
(refer to its atomic number on the periodic table). Beginning at the top of the
figure, we see that the first two electrons will fill the 1s sublevel to give 1s 2 , leaving
25 electrons. The next two electrons will enter the 2s sublevel giving 1s 2 2s 2 ,
leaving 23 electrons. Next is the 2p sublevel, which uses six electrons to give
1s 2 2s 2 2p 6 and leaving 19 electrons. After the 2p is the 3s sublevel, which takes
two electrons, 1s 2 2s 2 2p 6 3s 2 , leaving 15 electrons. Filling the 3p sublevel gives
1s 2 2s 2 2p 6 3s 2 3p 6 with nine electrons to go. The 4s sublevel follows the 3p, giving
1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 and leaving seven electrons. Finally, we reach the 3d sublevel,
which can hold 10 electrons. Since we only have seven electrons remaining,
the 3d sublevel will not fill. The ground-state electron configuration for
cobalt is: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 7 .
There are a few exceptions to this general pattern. The two best-known exceptions
are the elements chromium, Cr, and copper, Cu. The electron configurations
for these two elements are:
Cr 1s22s22p63s23p64s13d5 Cu 1s22s22p63s23p64s13d10 In both cases, the 4s sublevel is not filled even though there are electrons in
the 3d sublevel. In a later chapter, we will see why these “exceptions” are
predictable.
In some cases, it would be convenient to condense the electron configuration.
In this condensed form, the electronic configuration of the previous
noble gas forms a core represented by the atomic symbol of the element
enclosed in brackets (i.e., [He] or [Ne]). The electrons added since the noble
gas, follow the noble gas core. For example, cobalt can be represented as an
argon core plus the 4s and 3d electrons. Thus, 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 7 becomes
[Ar]4s 2 3d 7 .