From: Paul Doherty (pauld@exploratorium.edu)
Date: Tue Jun 28 2005 - 19:57:30 PDT
Message-Id: <f31f2a9b1cacbc39551e844c911a468a@exploratorium.edu> From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole transition metal ions electron configuration Date: Tue, 28 Jun 2005 19:57:30 -0700
Hi Ellen
I love what you do here.
In answer to the last question, the particle model electron falls in
toward the nucleus under the electrical attraction to the protons, it
misses the protons and then continues out the other side.
Or, Like the moon orbits the earth the electron orbits the nucleus.
With the difference that the electron's wave nature has more effect
than the wave nature of the moon.
Paul D
In answer to your last question
On Jun 28, 2005, at 5:51 PM, Ellen Koivisto wrote:
> Hi Geoff etc.
>
> Just got over a summer bug -- aren't they lovely -- hence my delay in
> following up on this topic. I love the explanation you got from the
> other listserv. I have a way of showing it to my kids that seems to
> work, though the focus is actually on understanding orbitals at all.
> It's called the dance of the electrons and takes a whole class, but
> it's worth it.
>
> In prep, I spend a lot of time over the entire year going over what
> models are, what they're good for, why we use them, and what they
> don't do. I tell them at the beginning of the year that, while we're
> learning about bonding, we'll use a model of the atom they all know
> (the electrons as planets orbiting the sun/nucleus) but that we know
> the model isn't accurate. We've done the 4 Forces by then so they can
> see that it can't work, but it's a good enough explanation until we
> get to quantum. They get comfortable with this provisional model for
> why things work, which is a major thing I try to teach them about
> science.
>
> Anyhow, once we hit quantum -- usually around January -- I introduce
> the whole weirdness in one fell swoop. As their little heads are
> bursting, I show them pictures of all the orbitals, not just the s and
> p that are in most textbooks. I particularly like Zumdahl's pix in
> the AP/college text, as he also includes cut-aways of s orbitals
> nested inside lower energy s orbitals, and he labels each orbital by
> its axial orientation. We go over how electrons in these orbitals fit
> inside each other as you get to higher and higher energy levels, and I
> do a wicked mime of how the electrons fill all the space available.
> As the energy levels get further away from the nucleus there is more
> space available, so more electrons can co-exist on the same energy
> level. They get it, pictures, explanations, teacher moving around,
> but it's still not solid until we go into the gym.
>
> Here we actually dance out a model of the electrons. We build slowly.
> A kid in the middle is the H proton, a kid circling is the H
> electron. Add another proton and another electron (we don't have
> space for neutrons, and even SF class sizes aren't big enough to from
> transition metals with correct proton/neutron/electron numbers.) The
> electrons quickly discover they can only move in 2 dimensions in any
> reasonable way (drilling through the floor or flying not being
> options). The protons have to jiggle and jostle, the electrons have
> to stay in motion. Anyhow, with electrical repulsion of like charges
> keeping the electrons going, it's clear that there isn't room for any
> more electrons at the 1s level. It's also clear that the s shape gets
> the electrons as close as possible to the nucleus.
>
> Next level out we get the 2s electrons (somewhere around carbon I stop
> adding proton students to the nucleus because it just gets too
> crowded, but we say we're adding another one every time), the s shape
> still being closest but requiring more energy to cover, but suddenly
> there's all this space at about the same level that doesn't have any
> electrons around for significant amounts of time. So we start adding
> in 2ps. Of course, we can't do the 2ps on the y axis (the flying
> problem again) but we talk through it when we hit it. Also about this
> time, the students see why each p orbital half fills before electrons
> are added to completely fill the orbitals. And they discover that
> even though they know the 2 electrons in the 2px, for example, can
> exist in either of the 2 nodule of this orbital, the students aren't
> capable of teleportation and so they tend to stay in one nodule each.
>
> We go on like this until we run out of students. I usually make it to
> 4s and some 3ds. My 1s electrons are very tired by the time we're
> done, so you can really have fun picking students for this one. Then
> they take a few minutes to write down notes before the class is over.
> A 1-page drawing with explanation of what we did and the short-comings
> of this model is due the next class. And it seems to work. They get
> how it works much better, and they don't have the problems with
> transition from model to experimental evidence or to different models
> that my students used to have before I thought this up.
>
> I still need some help making this work better though. I can tell my
> students that an electron acts both like a particle and a wave, and
> that the wave nature explains the nodes where no electrons are, but
> justifying that in terms of what they know about electrical attraction
> between the electrons and protons is where I'm stuck. Anyone have a
> good, math-free model that explains why the electrons don't dive into
> the nucleus and just stay there? Especially the 1s electrons, as
> these are unshielded.
>
> Thanks,
> Ellen Koivisto
> SF, SOTA
>
>
> On Jun 28, 2005, at 5:00 PM, Geoff Ruth wrote:
>
>> 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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