5.3. Electron Arrangement in Atoms http://www.ck12.org
Again, the number of valence electrons increases from one to eight across the third period.
Fourth and Fifth Period Elements
The element potassium begins the fourth period. The last electron in the potassium atom goes into the 4ssublevel,
which fills before the 3dsublevel. From this point onward, it is important to consult the diagram above (Figure
5.19) in order to follow the Aufbau process correctly. The fourth period elements are shown in the table below (
Table5.5).
TABLE5.5: Electron Configurations of Fourth-Period Elements
Element Name Symbol Atomic Number Electron Configuration
Potassium K 19 [Ar] 4 s^1
Calcium Ca 20 [Ar] 4 s^2
Scandium Sc 21 [Ar] 3 d^14 s^2
Titanium Ti 22 [Ar] 3 d^24 s^2
Vanadium V 23 [Ar] 3 d^34 s^2
Chromium Cr 24 [Ar] 3 d^54 s^1
Manganese Mn 25 [Ar] 3 d^54 s^2
Iron Fe 26 [Ar] 3 d^64 s^2
Cobalt Co 27 [Ar] 3 d^74 s^2
Nickel Ni 28 [Ar] 3 d^84 s^2
Copper Cu 29 [Ar] 3 d^104 s^1
Zinc Zn 30 [Ar] 3 d^104 s^2
Gallium Ga 31 [Ar] 3 d^104 s^24 p^1
Germanium Ge 32 [Ar] 3 d^104 s^24 p^2
Arsenic As 33 [Ar] 3 d^104 s^24 p^3
Selenium Se 34 [Ar] 3 d^104 s^24 p^4
Bromine Br 35 [Ar] 3 d^104 s^24 p^5
Krypton Kr 36 [Ar] 3 d^104 s^24 p^6Beginning with scandium (Z= 21), the 3dsublevel begins to fill. Note that it is customary for the lower principal
energy level (the 3rd) to be written first in the electron configuration even though it fills after the outer (4th) principal
energy level. Titanium and vanadium follow scandium. We would expect that the next element, chromium, would
have an outer configuration of 3d^44 s^2. However, in this case, one of the 4selectrons is shifted to the last of the
empty 3dorbitals, resulting in an outer configuration of 3d^54 s^1. This has a slightly lower energy because having six
unpaired electrons instead of four minimizes electron-electron repulsions. As a result, the ground state of chromium
has a different arrangement than the one we might expect based on the Aufbau principle.
Proceeding onward, manganese (Mn) pairs up the single electron in the 4sorbital. Iron (Fe), cobalt (Co), and nickel
(Ni) follow, with electrons now pairing up in the 3dorbitals. Copper (Cu) is another element with an unexpected
configuration. This time, the second 4selectron is shifted into the last available spot in the 3dsublevel, completely
filling it. This is the lowest energy configuration for Cu. Electron configurations that do not quite follow the usual
order of filling orbitals, like those of Cr and Cu, occur frequently among the heavier elements, and such exceptions
are not always easy to explain. It is important to be aware of them, but it is not necessary to memorize every
exception. From gallium (Ga) through the noble gas krypton (Kr), electrons fill the 4psublevel in a straightforward
manner.
Recall that valence electrons are only those electrons in the outermost principal energy level. For elements 21-30,
electrons are being added to the 3dsublevel. However, the 3dsublevel is not the outermost energy level, because