Orbital Notation3 d 4 s 4 p Simplified Notation19 K[Ar] __h [Ar] 4s 120 Ca [Ar] __hg [Ar] 4s 221 Sc[Ar]__h hg [Ar] 3d (^14) s 2
22 Ti [Ar]
hh hg [Ar] 3d (^24) s 2
23 V[Ar]
hhh hg [Ar] 3d (^34) s 2
24 Cr [Ar]
hhh hh h [Ar] 3d (^54) s 1
25 Mn [Ar]
hhh hh hg [Ar] 3d (^54) s 2
26 Fe [Ar]
hghh hh hg [Ar] 3d (^64) s 2
27 Co [Ar]
hghg h hh hg [Ar] 3d (^74) s 2
28 Ni [Ar]
hghg hg hh hg [Ar] 3d (^84) s 2
29 Cu [Ar]
hghg hg hg hg h [Ar] 3d (^104) s 1
30 Zn [Ar]
hghg hg hg hg hg [Ar] 3d (^104) s 2
31 Ga [Ar]
hghg hg hg hg hg h [Ar] 3d (^104) s (^24) p 1
32 Ge [Ar]
hghg hg hg hg hg hh [Ar] 3d (^104) s (^24) p 2
33 As [Ar]
hghg hg hg hg hg hh h [Ar] 3d (^104) s (^24) p 3
34 Se[Ar]
hghg hg hg hg hg hg h h [Ar] 3d (^104) s (^24) p 4
35 Br [Ar]
hghg hg hg hg hg hg hg h [Ar] 3d (^104) s (^24) p 5
36 Kr [Ar]
hghg hg hg hg hg hg hg hg [Ar] 3d (^104) s (^24) p 6
As you study these electron configurations, you should be able to see how most of them
are predicted from the Aufbau order. However, as we fill the 3dset of orbitals, from 21 Sc
to 30 Zn, we see that these orbitals are not filled quite regularly. As the 3dorbitals are
filled, their energies get closer to that of the 4sorbital and eventually become lower. If
the order of filling of electrons on chromium gave the expected configuration, it would
be: [Ar] 4s^23 d^4. Chemical and spectroscopic evidence indicates, however, that the config-
uration of Cr has only one electron in the 4sorbital, [Ar] 4s^13 d^5. For this element, the 4s
and 3dorbitals are nearly equal in energy. Six electrons in these six orbitals of nearly the
same energy are more stable with the electrons all unpaired, [Ar] 3dhhhhh 4 sh
rather than the predicted order [Ar] 3d h__hhh 4 s__hg.
The next elements, Mn to Ni, have configurations as predicted by the Aufbau order,
presumably because forming a pair of electrons in the larger 4sorbital is easier than in a
smaller, less diffuse, 3dorbital. By the time Cu is reached, the energy of 3dis sufficiently
lower than that of 4s, so that the total energy of the configuration [Ar] 4s^13 d^10 is lower
than that of [Ar] 4s^23 d^9.
We notice that the exceptions for Cr and Cu give half-filled or filled sets of equiva-
lent orbitals (d^5 and d^10 , respectively), and this is also true for several other exceptions to
the Aufbau order. You may wonder why such an exception does not occur in, for example,
14 Si or 32 Ge, where we could have an s^1 p^3 configuration that would have half-filled sets
of sand porbitals. It does not occur because of the very large energy gap between nsand
nporbitals. There is some evidence that does, however, suggest an enhanced stability of
half-filled sets of porbitals.
End-of-chapter Exercises 81–111
provide much valuable practice in
writing electron configurations.
Problem-Solving Tip:Exceptions to the Aufbau Order
In Appendix B, you will find a number of exceptions to the electron configurations
predicted from the Aufbau Principle. You should realize that statements such as the
Aufbau Principle and the (n1) rule merely represent general guidelines and should
The electron configurations of
elements 1 through 109 are given in
Appendix B.