From: Geoff Ruth (gruth@leadershiphigh.org)
Date: Tue Jun 28 2005 - 17:00:03 PDT
Message-Id: <p06110409bee794e1b77a@[192.168.1.103]> Date: Tue, 28 Jun 2005 17:00:03 -0700 From: Geoff Ruth <gruth@leadershiphigh.org> Subject: Re: pinhole transition metal ions electron configuration
I got an answer that I like a lot from another listserv to my prior
question. I'm posting it below to share:
First of all, the order we traditionally list is what I call
the FILLING
ORDER, which is the order in which electrons FIRST enter the orbitals.
This is no guarantee that that continues to be the energy order of the
orbitals after electrons are present. This order is affected by (as Mike
mentioned) the number of electrons in the orbitals. Remember that the
orbitals are different sizes, and the size is related most closely to the
principal quantum number, n. By size, we are referring primarily to the
distance away from the nucleus with the greatest probability of finding
the electrons. Specifically, the 4s orbital is larger than the 3d
orbitals. In general, the larger the orbital, the higher its energy.
However, there is another factor that influences the INITIAL
energy of an
orbital, and that is the number of nodes the orbital has. A node is a
"line/surface" where the likelihood of finding an electron is zero. The
greater the number of nodes, the higher the energy of the orbital. This is
similar to the number of nodes in a standing wave on a spring or a rope.
The more nodes, the more energy is needed to produce the standing wave.
There are two types of nodes in an orbital: Radial nodes (at a certain
distance away from the nucleus) and angular nodes (at certain directions
from the nucleus). The number of radial nodes is related to n, but the
number of angular nodes is related to the azimuthal quantum number, l. The
4s has more radial nodes than the 3d's, but the 3d's have two angular
nodes (as ALL d orbitals do) where the 4s has none (like all s orbitals).
This gives the 3d orbitals more energy initially than the 4s orbitals.
All bets are off, however, when we start placing electrons in the
orbitals. As noted above, the 3d's are closer to the nucleus, so there is
more attraction between 3d electrons and the nucleus than between 4s
electrons and the nucleus (simple physics here: the force is inversely
related to the distance between the two charges). This makes it harder to
pull the 3d electrons away from the nucleus. In addition, chemists like to
talk about "shielding" by inner electrons reducing the "effective nuclear
charge" on the outer electrons. As soon as we start adding electrons to
the 3d orbitals, the relative energies of the subshells change (as Mike
noted), so that, before we have added very many electrons to the 3d
subshell (2?) the 3d subshell has lower energy than the 4s subshell and
the first electrons removed come from the 4s rather than the 3d.
I hope this helps.
Tom Harrison
Sandy Spring Friends School
Sandy Spring, MD
tomh@ssfs.org
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