Chemistry Reference
In-Depth Information
quantum number. In fact, the presence of these nodes shows that the ns orbitals
are composed of concentric layers like an onion, and that at least one electron is
much closer to the nucleus than the electron on the 2p orbital. Therefore, the 2s
orbitals achieve more penetration than the 2p orbitals. In fact, the s orbitals are
much more penetrating than the p or d of the same layer, that is, an electron on an
s orbital has a higher probability of being near the nucleus than an electron on a
p or d orbital. This is also the reason that s electrons have a higher shielding power
than the electrons of other orbitals. It must be emphasized that the relative ener-
gies of the ns atomic orbitals are smaller than the energies of the corresponding
(n − 1)d orbitals.
Together, shielding and penetration phenomena, and the laws of Hund and
Pauli, provide a partial explanation for the way the orbitals are filled by electrons.
In a more simplistic way, Kleshkowski formulated the rule of the minimum n + l
sum that says, “the subshells are being arranged in ascending order of the n + l
sum, and for equal values of the sum, in an order corresponding to increasing
principal quantum number n” (Wong 1979). Thus, if we write the sequence of
symbols of the subshells arranged in ascending order of n and l, and the value of
the n + l sum, we get:
n
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
5g
6s
6p
6d
..
7s
7p
..
n + l
1
2
3
3
4
5
4
5
6
7
5
6
7
8
9
6
7
8
..
7
8
With Kleshkowski's law we obtain the real order in which electrons fill the atomic
subshells: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p…. This
law is illustrated in
Figure 2.5
where the order for filling the orbitals of an atom is
indicated by the orientation of the arrow.
If the maximum number of electrons in orbitals is included, the order for filling
the orbitals of an atom can be written as follows:
1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
10
4p
6
5s
2
4d
10
5p
6
6s
2
4f
14
5d
10
6p
6
7s
2
5f
14
6d
10
…
where the figure in front of the orbital is the principal quantum number and the
superscript represents the number of electrons in the orbital. There are deviations
from this rule. For example, known exceptions include lanthanum and actinium.
Thus, before the filling with electrons of the 4f subshell, 5f subshell, respec-
tively, the distinctive electron of La (respectively Ac) is placed on an orbital of
the 5d subshell (respectively 6d subshell). Such exceptions also occur in the other
d-block elements. Spectroscopic data show that d-block elements have the form
3d
n
4s
2
with the 4s orbitals filled, but chromium has the electronic structure 1s
2
2s
2
2p
6
3s
2
3p
6
4s
1
3d
5
instead of 1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
4
. The energy decreases by
putting electrons in odd orbitals. Such behavior is also found in copper, silver, and
gold atoms.
It is obvious from the above that the orbital energies, or more precisely the order
by which orbitals are filled, depends on the shielding effect, the effect of electron
penetration, and the filling level of the neighboring orbitals.
