Energy of excited electrons
Compilation. Unit 11: Energy of excited electrons
Date: Thu, 20 Apr 2006
From: Tom Pfeiffer, Bellows Free Academy, Vermont
Subject: electrons moving between energy levels
We've arrived at unit 11 and we've just introduced the Bohr theory after looking at the
spectra of various light (incandescent, fluorescent and gas tubes). We discussed electrons
occupying discrete energy levels and emission of energy when electrons move from excited
states to lower energy levels. I was thinking of our energy bar graphs and flow diagrams we've
used in previous units. We could represent energy flowing out of the atom via radiating (R). The
bar charts are a little trickier because I'm not sure how to represent the account that the energy
would be in when the electron is in an excited state.
I would think it would be something like potential energy, perhaps chemical potential
energy. That seems to make sense to me especially when I think of the electron transport system
in mitochondria. The high energy electrons are passed along to various cytochromes and transfer
energy to NAD. Has anyone else thought about this one?
-------------------Date: Fri, 21 Apr 2006
From: John Barrere
Wouldn't it be more accurate to call this electric potential energy?
-------------------Date: Fri, 21 Apr 2006
From: Carmela Minaya, Hanalani Schools
So two energy bar graphs are needed: one for when the electron jumps to a higher shell
getting excited by radiating into and the other for when the energy is released by radiating out
when it drops back down to the lower level. When it is excited, it is physically being transported
to a higher shell so it's moving. I think that's Ek. Or was the motion just to get it into the higher
shell? Isn't it still Ek then? Am I thinking wrong? How would that work in reverse?
I actually brought this one up in my AP chemistry class just now, and it might make
sense to have energy entering the system first step. That energy is first stored in Ek then within
the system is transferred to Ech for availability to react with other particles; that's the second step.
The reverse would be a transfer within the system from Ech account into the Ek account
(1st step) and out of the system subtracting from the Ek account (2nd step). Clear as mud? What
do you think? This came from one of my brightest thinkers. This is something like what we did
for endothermic/exothermic reactions. Remember the LOLOL diagrams?
-------------------Date: Mon, 24 Apr 2006
From: Guy Ashkenazi
I always had a problem with the titles kinetic energy and potential energy when applied
to chemical systems (they are perfectly fine when applied to mechanical systems). I prefer
calling them thermal energy and chemical energy. There are two reasons for that:
1. Changes in mechanical kinetic and potential energy are not associated with entropy
change, while changes in thermal and chemical energies are both associated with entropy
changes. This makes thermal and chemical energies behave quite differently than mechanical
kinetic and potential energies (for example, mechanical equilibrium is temperature independent,
while chemical equilibrium is temperature dependent).
2. Thermal energy has a potential component in it - the potential energy of vibration
(actually in a metal half of the thermal energy is stored as potential energy and only half as
kinetic). Chemical energy has a kinetic component in it - the kinetic energy of the electrons (you
1
Energy of excited electrons
are invited to read my paper in the last issue of The Chemical Educator about the importance of
electronic kinetic energy in the formation of a stable covalent bond).
If we change from the language of kinetic and potential to the language of thermal and
chemical, Tom's problem is solved easily. Since the energy change is stored in a specific change
in structure (electronic structure), and not in a change in random motion, it is a change in
chemical energy, and not in thermal energy.
-------------------Date: Tue, 25 Apr 2006
From: Carmela Minaya
Thanks for the reminder. I remember you talking about this last summer. I will try to
remember to use this language next time. It sure takes a lot of work to align energy thinking in
an appropriate manner. Does this mean in a phase change rather than storing energy in the
interaction energy container, we would call all potential energy containers chemical energy? I
don't see a problem with that. I just want clarification for when it's not the bond, but the
intermolecular forces that are affected.
-------------------Date: Tue, 25 Apr 2006
From: thomas pfeiffer
So the energy change due to change in electronic structure refers to changes in energy
levels. And this is quantized, limited by allowed energy levels and transitions, right?
Is the energy due to random motion kinetic? Is this energy "quantized", meaning it can
only have certain amounts, like the energy described above?
-------------------Date: Thu, 27 Apr 2006
From: Guy Ashkenazi
Yes, it is quantized.
The energy due to random motion is purely kinetic in an ideal monatomic gas. When
atoms are interacting, part of the thermal energy would be stored in vibrations, which are half
kinetic - half potential on the average (energy changes back and forth between the two during the
vibration).
Vibrational and rotational energies are quantized (as observed in IR vibration/rotation
spectrum of molecules). However, at high enough temperatures, the separation between energy
levels is so small compared to the total energy, that for all practical purposes you can treat it as
continuous.
For the translation, all temperatures are high enough. For rotations, room temperature is
high enough (except if the rotating atom is hydrogen). For most vibrations, room temperature is
borderline - the vibrations are excited, but not enough to form a continuum. For very strong
bonds, such as the triple bond in N2, room temperature is so low that most molecules are at the
ground vibrational level, and can be considered as rigid (not vibrating at all).
Most metals can be considered as having a continuum of vibrational energy at room
temperature, therefore most metals have a heat capacity of 3R (8.31 J/mol-K per vibrational degree
of freedom), regardless of binding strength.
2