Exam 2 Thursday (7:30-9pm)
It will cover material through HW 7, but no material
that was on the 1st exam.
What happens if we bash atoms with
electrons?
In atomic discharge lamps, lots of electrons are given kinetic
energy (accelerated by a high voltage). When they bash into
atoms some of this kinetic energy is transferred to the atom
à Atom get's excited!! (“Neon” lights, Mercury street lamps)
- 120V +
Cathode (hot metal, so
electrons can come out)
Anode (positive potential)
Note: ‘Anode’ and ‘Cathode’ have different meanings in physics or
chemistry. Remember ‘Cathode Ray Tube’ (CRT): Electrons
leave the cathode (in physics).
Use a grating to look at the spectrum of
the discharge lamps
Hold grating only by edges...oil from hands ruins grating!
Hold close to eye... See rainbow from lights. Turn grating
so rainbow is horizontal. (Rainbow appears quite a bit to
the side of the actual lamp.
Each type of atom produces unique set of colors.
Discussion: What does this imply about electrons in atoms?
Implies that electrons only change between very specific energies.
Each time a photon is emitted an electron must be changing in
energy by that amount (releasing energy).
Only way for individual atoms to give off energy is as light.
Atoms are lazy - always want to go back to lowest energy state.
1.  Fast electron
hits atom
Less KE
e
Higher
energy
e
Ground
state
~10ns
Excited
state
e
What we know so far about atoms:
Rutherford: Atoms have a tiny, but very dense
core surrounded by a cloud of electrons.
p
n nn
p p p
nn nn
p
Discharge lamps: Atoms struck by fast
electrons emit light of distinct colors.
120V
Energy levels: Electrons in atoms are found only
in discrete energy levels. When they jump
down to a lower level, a photon is emitted
carrying away the energy.
ΔE
ΔE=hf
e
Different Atoms: Different types of atoms have different level
structures (seen by the distinct set of colors they emit):
àNeon: Strong red line; Sodium: strong yellow line …
A neon lamp emits a red line. Sodium emits a yellow line.
What accounts for this difference?
a. The electrons in the discharge lamp hit the neon atoms
with higher kinetic energy than the electrons hit the sodium
atoms
b. The electrons in the discharge lamp hit the neon atoms
with less speed than the electrons hit the sodium atoms
c. The energy spacing between the electronic energy levels
that are responsible for these lines are smaller in the neon
atom than in the sodium atom.
d. The energy spacing between the electronic energy levels
that are responsible for these lines is larger in the neon
atom than in the sodium atom.
e
e
e
e
Energy
Energy level diagrams represent energy levels the electron can
go to. Different height = different energy
e
For Hydrogen,
transitions to
ground state in
deep ultraviolet!
No light emitted with
colors in this region
because no energy
levels spaced with
this energy.
e
An single electron bashes into an atom in a discharge lamp
Electron leaves hot filament with
nearly zero initial kinetic energy
-2 eV
- 10 V +
-3 eV
-6 eV
-10eV
If atom fixed at the center of the tube,
list all the possible photon energies (colors) that you might see?
A. 1eV, 2eV, 3eV, 4eV, 7eV, 8eV
B. 4eV, 7eV, 8eV
C. 1eV, 3eV, 4eV
D. 4eV
E. Impossible to tell.
An single electron bashes into an atom in a
discharge lamp
- 10 V +
d
D
-2 eV
-3 eV
-6 eV
If atom fixed at this point in tube,
-10eV
list all the possible photon energies (colors) that you might see?
A. 1eV, 2eV, 3eV, 4eV, 7eV, 8eV
B. 4eV, 7eV, 8eV
C. 1eV, 3eV, 4eV
D. 4eV Answer is D. Electron only gains about 5eV!
E. Impossible to tell.
Electron energy = qΔV = e(Ed),
where E is the electric field = (battery V)/(total distance D),
and d is the distance it goes before a collision.
Remember the ‘Electron Volt’ (eV)?
Define new energy unit: The electron-volt (eV)
1eV = kinetic energy gained/lost by an electron when
accelerated/decelerated by traversing an
area with 1 Volt potential difference (such as in
a plate capacitor)
1eV ≈ 1.6 ·10-19 J
0V
F
E
path
1V
+
+
+
+
Some handy relations:
Current flowing through a wire:
1 Ampere = 1 Coulomb / Second
1 Watt = 1 Ampere · 1 Volt = 1 C/s · 1 V
1 Joule = 1 Watt · 1 Second = 1 C · 1 V
1C
0V
i.e: An electric potential of 1 V puts in one Joule
of energy into a charge of 1 C àkinetic
energy increases by 1 Joule.
1V
+
+
+
+
An e- has a charge of 1.6·10-19C,
therefore, it gains a kinetic energy of
1eV=1.6·10-19J
Investigate energy levels in atoms
energy levels of
electron in atom
3
2
1
energy
of colliding
electron
G (ground)
If the colliding electrons have an energy between that of level 2
and level 3 when they hit the atom how many different colors
could be emitted by the atom?
a.  no levels will be excited, and so no light will come out.
b. 1 color of light will come out
c. 2 colors of light will come out
d. 3 colors of light will come out
e. 4 colors come out.
ans. d. enough energy to excite level 2, then get 2⇒1 followed
by 1⇒G, but also can go 2⇒G.
When an emission line appears very bright that
means:
a. Multiple photons are emitted for each single
electron transition
b. More electron transitions are occurring each
second
c. The electronic energy levels are farther apart
and thus the line appears brighter
d. a and b
Answer is b. Each electron transition
e. b and c
produces exactly one photon with energy
equal to energy difference between electron
energy levels. More electron transitions,
more photons, brighter line.
Applications of atomic spectroscopy
1. Detecting what kind of atoms are in a material.
(excite by putting in discharge lamp or heating in flame
to see spectral lines)
2. Detecting what the sun and the stars are made of:
Look at the light from a star through a diffraction grating.
àSee what lines there are; Match up to atoms on earth.
telescope
star
diffraction
grating
3. Making much more efficient lights!
Incandescent light bulbs waste >90% of the electrical
energy that goes into them! (<10% efficient)
Streetlight discharge lamps (Na or Hg) ~80%
efficient. Fluorescent lights ~ 40-60% efficent.
Atomic spectra in astronomy
L
Let’s investigate the internal
working of a galaxy
L
R
R
longer wavelength
lower frequency
Spectral lines from
atoms in stars
Hubble and the big bang
Spectral lines from Hydrogen
Edwin Hubble, PNAS March 15, 1929 vol. 15 no. 3 168-173
Application:
Designing a better light
What is important?
(Well, we have to be able to see the light!)
What color(s) do you want?
(red, green & blue is what our eyes can see)
How do you excite the atoms to desired level?
(à Electron collisions)
How to get electrons with desired energy when
hit atoms? What determines energy of electrons?
Incandescent light (hot filament)
Temperature = 2500-3000K
Hot electrons jump
between many very
closely spaced levels
(solid metal). Produce all
colors. Mostly infrared at
temp of normal filament.
>90% is worthless
Infrared radiation (IR =
longer than ~700nm)
IR
P
λ
~10% of energy is
useful visible light
Discharge lamp
Energy levels in
isolated atom:
Only certain
wavelengths
emitted.
HV
Right atom, right pressure
and voltage, mostly visible light.
Streetlight discharge lamps
(Na or Hg) 80% efficient.
Florescent Lights.
Discharge lamp, but want to have it look white.
White = red + green +blue
40-60% efficient (electrical power visible light)
How to do this?
Converting UV light into visible photons with “phosphor”.
Phosphor converts 180 nm UV to red+ green+blue.
180nm à 6.9 eV energy per photon
633nm (red) à 2 eV / photon
532nm (green) à 2.3 eV / photon
475nm (blue) à 2.6 eV / photon
phosphor wastes 20-30% energy ⇒ heat
}
6.9 eV
Florescent Lights. Discharge lamp, White= Red + green +blue
40-60% efficient (electrical power visible light)
Converting 180nm UV light into visible photons with “phosphor”.
phosphor
coating
Hg
180 nm far UV
Hg
Hg
Hg
Hg
e
Hg
discharge lamp/flor. tube
120 V
real phosphors more than just 3
phosphor wastes 20-30% energy
⇒ heat
energy of electron
in phosphor molecule