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The Bohr Atom - Evolution of Electron Configuration
The Bohr Atom Evolution of Electron Configuration
2
The Bohr Atom - Evolution of Electron Configuration
Model
Observations
Theory
Learning about the Electron
Technicians, engineers and scientists, know a lot about electrons today because of a three pronged cyclic learning process.
> Conduct experiments (observations) to explore a phenomena. > Develop a story (theory) to predict what will happen in the next experiment.
> Find equations (i.e., models) that explain and support the theory.
This repetitive process continues as the knowledge base increases.
3
The Bohr Atom - Evolution of Electron Configuration
Dalton’s “Atomic” Theory (1808)
J. J. Thomson’s “Plum Pudding” Model (1898)
Ernest Rutherford’s “Nucleus” Model (1909)
Aristotle’s “Composition of the Universe” Theory (354 BC)
All things are composed of earth, air, fire, and water with no concept of an electron.
Tiny indivisible particles that have no sub-particle sized parts.
Sub-particle sized parts exist in an atom but no idea of their arrangement.
Electrons are independent sub-particle sized parts occupying their own specific space.
Learning about the Electron
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The Bohr Atom - Evolution of Electron Configuration
Electrons orbit the nucleus in different orbits at different fixed distances.
Electrons only change orbits if specific amounts (quanta) of extra energy from the outside world are involved. Electrons that receive enough extra energy from the outside world can leave the atom they are in.
Electrons that return to orbits they used to reside in give up the extra energy they acquired when they moved in the first place.
Electronic energy given up when electrons move back into an original orbit often emit a specific color light.
Electrons that leave one orbit must move to another orbit.
Summary of Bohr’s (Planetary) Model (1913)
Learning about the Electron
5
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve Model
Obse
rvati
ons
Theory
ObservationsThe most important observation Bohr needed to explain was the fact that when energy was added to atoms the atoms gave off (emitted) light.
TheoryThe biggest problem with Rutherford’s story was why the electrons don’t eventually crash into the nucleus and destroy the atom.
ModelUp to this point in time (1913), nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms. A difficult puzzle to solve, but Bohr made a major step in putting the pieces together.
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The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
ObservationsWhen an element is subjected to heat
Thermal Emission (Max Plank 1900)> A hot solid will emit
light. > A solid glows red at 750 degrees centigrade. > A solid glows white as the temperature
increases to 1200 degrees centigrade.
As the temperature increases from 750 degrees, more yellow and blue light is emitted and mixes with the red light to give white light.
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
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The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
Observations
Light Source
Photons
Photoelectric EffectAlbert Einstein (1905)
(Nobel prize 1921)
Light (photons) can force electrons to be emitted from the surface of a metal.
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
Photoelectric Effect
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The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
ObservationsWhen an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
Corona
Who, What, Where is it?
Who
St. Elmo’s Fire
What/Where
As described in “The Tempest”
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
9
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
ObservationsWhen an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
“I boarded the king’s ship; now in the beak,Now in the waist, the deck, in every cabin, I flamed amazement; sometimes I’d divideAnd burn in many places; on the topmast, The yards and bowsprit, would I flame distinctly,Then meet and join.”A passage from “The Tempest” by William
Shakespeare
Corona (1611)
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The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
Bohr’s postulates include the belief that
Orbit number 1 (n = 1)
Orbit number 3 (n = 3)
Observations
electrons that leave one orbit must move to another orbit.
electrons are in orbits but the orbits are at different distances from the nucleus.
Ernest Rutherford’s Model (1909)
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The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
2 electrons in orbit number 1
2 electrons in orbit number 3
Observations
C
6
12
Atom with 6 protons and 6 electrons
Bohr’s concept of a carbon atom with a “ground” state electron configuration.
“Ground” state electron configuration
remember an atom that has 6 protons is always called a carbon atom
2 electrons in orbit number 2
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
12
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
Observations
electrons only change orbits if specific amounts (quanta) of extra energy from the outside world are involved.
C
6
12 Atom with 6 protons and 6 electrons
“Ground” state electron configuration“Excited” state electron configuration
The electron configuration of an atom changes when energy (heat, light, or electricity) is added to the atom.
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
13
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Observations
Energy is removed from the atom when an electron returns to its original orbit.
C
6
12 Atom with 6 protons and 6 electrons
“Excited” state electron configuration“Ground” state electron configuration
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
Obse
rvati
ons
14
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
Observations
electrons only change orbits if specific amounts (quanta) of extra energy from the outside world are involved.
this electron used to be in orbit #2
add energy
“Excited” state electron configuration
“Higher” energy state for an atom
“Ground” state electron configuration“Lower” energy state for an atom
C
6
12
15
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
Observations
electrons only change orbits if specific amounts (quanta) of extra energy from the outside world are involved.
“Excited” state electron configuration
“Higher” energy state for an atom
When an electron returns to an orbit it use to be in it will always give up a quanta of energy. Often this energy is put back into the world as a photon of colored light.
release energy
“Lower” energy state for an atom
“Ground” state electron configuration
this electron has returned to orbit #2C
6
12
16
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
Observations
“Ground” state electron configuration
When an element is subjected to heat,
Sometimes, the amount of energy that is taken up by the atom is the exact amount that will let an electron in the atom’s outer orbit actually leave the atom.
Light Source
Photons
Photoelectric Effect
N
7
14
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
17
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Obse
rvati
ons
ObservationsWhen an element is subjected to heat, light,
If atoms are located in an electric field, The electrons can get the exact energy they need from the field to move to an outer orbit.
When electrons give up that energy and return to their original orbits, light is emitted.
positive electrode
negative electrodeCorona and Plasma Discharges
When an element is subjected to heat, light, or electrical discharge, When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
18
The Bohr Atom - Evolution of Electron Configuration
Obse
rvati
ons
positive electrode
negative electrodeHelium corona and plasma discharge
The Puzzle Pieces Bohr had to Solve
ObservationsWhen an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light.
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The Bohr Atom - Evolution of Electron Configuration
Theory
Electrons remain in a specific orbit unless they obtain additional energy to move into orbits that are further from the nucleus.
Electrons can only have a specific energy value to be in a specific orbit of an atom.
Therefore, electrons are not satellites (Rutherford’s theory) like man made satellites in continuous decaying orbits that gradually spiral toward the center.
Model
Obse
rvati
ons
TheoryTheory
The biggest problem with Rutherford’s story was why the electrons do not eventually crash into the nucleus and destroy the atom.
The Puzzle Pieces Bohr had to Solve
Bohr’s postulates include the belief that
20
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Theory
Theory
The biggest problem with Rutherford’s story was why the “satellite” electrons don’t eventually crash into the nucleus and destroy the atom.
Bohr’s Energy Level Postulates:Orbit number 1 is the orbit closest to the nucleus.
Each electron in orbit number 1 has
2.18 x 10 -18 Joules of energy
(This is a tiny amount of energy, but remember that an atom is a tiny thing in the first place.)
21
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
Theory
Theory
Energy electron needs
2.18 x 10
-18Joules
Orbit number
1
0.55 x 10
-18Joules
2
0.24 x 10
-18Joules
3
The biggest problem with Rutherford’s story was why the “satellite” electrons don’t eventually crash into the nucleus and destroy the atom.
The energy of an electron orbit is inversely proportional to the square of the orbit number.
22
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelUp to this point in time, nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms.
Model
Obse
rvati
ons
Theory
Model
Bohr’s Energy Level Equation Postulate
electron energy level
equals Joules
22.18 x 10
-18
(orbit number)
The energy of an electron orbit is inversely related to the square of the orbit number.
E Joules
2.18 x 10
-18
2(n)
=
The energy of an electron orbit is inversely related to the square of the orbit number.
23
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelUp to this point in time, nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms.
Model
Bohr’s Energy Level Equation Postulate
n =1
E Joules
2.18 x 10
-18
2(n)
=n
The Bohr model calculated energy for orbit the closest to nucleus.
2.18 x 10
-18
(n)(n)2.18 x 10
-18Joules
2.18 x 10
-18
(1)(1)=
E 1 ==
The Bohr model for calculating the energy relative to an atomic orbital.
24
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
E Joules
2.18 x 10
-18
2(n)
=n
The Bohr model for calculating the energy relative to an atomic orbital.
The Bohr model calculated energy for second (n=2) orbit. n =2 E
2
=
2.18 x 10
-18
(2)(2)0.55 x 10
-18Joules
2.18 x 10
-18
4=
=
Up to this point in time, nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms.
25
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
E Joules
2.18 x 10
-18
2(n)
=n
The Bohr model calculated energy for third (n=3) orbit. n =3 E
3
=
2.18 x 10
-18
(3)(3)
2.18 x 10
-18
90.24 x 10
-18Joules
=
=
The Bohr model for calculating the energy relative to an atomic orbital.
Up to this point in time, nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms.
26
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
third (n=3) orbit,
E 3
=
2.18 x 10
-18
(3)(3)
2.18 x 10
-18
90.24 x 10
-18Joules
=
=
The Bohr model energy calculation for the:second (n=2) orbit,
E 2
=
2.18 x 10
-18
(2)(2)0.55 x 10
-18Joules
2.18 x 10
-18
4=
=
and the fourth (n=4) orbit.
E 4
2.18 x 10
-18
160.14 x 10
-18Joules
=
=
27
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
The Bohr model energy calculation forsecond (n=2) orbit.
E 2
=
2.18 x 10
-18
(2)(2)0.55 x 10
-18Joules
2.18 x 10
-18
4=
=
energy difference for electron transitions between second and fourth orbits. 0.14 x 10
-18Joules= 0.55 x 10
-18Joules -
fourth (n=4) orbit E 4
2.18 x 10
-18
160.14 x 10
-18Joules
=
=
0.14 x 10-18
0.55 x 10-18
0.55 x 10
-18
0.14 x 10
-18
= E2 – E4
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The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
energy difference for electron transitions
between second and fourth orbits.
Joules
= - 0.14 x 10
-180.55 x 10
-18[ ]
energy difference
Joules
0.41 x 10
-18=
If Niels Bohr were alive today, he would say that when an electron moves from the fourth orbit of an atom to the second orbit of the same atom, then
Joules
0.41 x 10
-18
of energy will be released.
29
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
Niels Bohr believed that
Joules
0.41 x 10
-18of energywould be released when an electron went from orbit 4 to
orbit 2 in an atom.
Now he just had to convince the rest of the world that what he believed was what really happened.
How do you think he did that?
30
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
Niels Bohr believed that
Joules
0.41 x 10
-18of energywould be released when an electron went from
orbit 4 to orbit 2 in an atom.
123
4
Bohr knew he could use Max Planck’s equation to connect the energy of a light wave to its frequency (color).
(1)
=fphoton
=Planck’s constant
energy released
h
Joules
0.41 x 10
-18
31
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
123
4
=fphoton
Joules
0.41 x 10
-18
h
=fphoton
Joules
0.41 x 10
-18
Joules/second
6.63 x 10
-34
Niels Bohr believed that
Joules
0.41 x 10
-18of energywould be released when an electron went from
orbit 4 to orbit 2 in an atom.
32
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
123
4
=fphoton
=fphoton
Joules
0.41 x 10
-18
Joules · second
6.63 x 10
-34
cyclessecond
-18
6.63
+34x 10
0.41
x 10
33
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
123
4
=fphoton
fphoton
= 0.062 x 10+16 cycle
ssecond
+16 cyclessecond
0.416.63
x 10
=fphoton
cyclessecond
-18
6.63
+34x 10
0.41
x 10
34
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
123
4
fphoton
= 0.062 x 10+16 cycle
ssecond
fphoton
= 6.2 x 10
+14 cyclessecond
35
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
Niels Bohr now believed that the
Joules
0.41 x 10
-18of energyreleased from an atom when an electron went from the
atom’s fourth orbit to its second orbit was a photon of light that had a frequency of
123
4
6.2 x 10
+14 cyclessecond
Niels Bohr also knew the relationship between the frequency of a light wave and its wavelength.
Do you remember that equation?
36
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
123
4
Shortest distance between the dotted lines is the wavelength of this wave.
length of a wave
=speed of light
frequency of light wave
Niels Bohr believed that
Joules
0.41 x 10
-18of energyis the energy associated with a
light wave that has a frequency of
6.2 x 10+14 cycles
second
37
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
123
4
=speed of light
6.2 x 10+14 cycle
ssecond
=6.2 x 10
+14 cyclessecond
+8 meters3.0 x 10 second
Niels Bohr believed that
Joules
0.41 x 10
-18of energyis the energy associated with a
light wave that has a frequency of
6.2 x 10+14 cycles
second
Shortest distance between the dotted lines is the wavelength of this wave.
length of a wave
38
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
123
4
=
=
6.2 x 10+14 cycle
ssecond
+8 meters3.0 x 10 second
6.2
-14+83.0 x 10
cycle
metersx 10
Shortest distance between the dotted lines is the wavelength of this wave.
length of a wave
=6.2
-63.0 x 10
cycle
meters
= -60.48 x 10
cycle
meters
39
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
123
4
=
=
cycle
meters
480 nanometers
480 x 10-9
=-6
0.48 x 10cycle
metersShortest distance between the dotted lines is the wavelength of this wave.
length of a wave
40
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
After all this work, Bohr now believed that a photon released from an atom when an electron went from orbit 4 to orbit 2 in the same atom would have the following characteristics:
Joules
0.41 x 10
-18
Energy
Frequency
Wavelength480
nanometers
6.2 x 10+14 cycle
ssecond
41
The Bohr Atom - Evolution of Electron Configuration
The Puzzle Pieces Bohr had to Solve
ModelM
odel
Joules
0.41 x 10
-18
6.2 x 10+14 cycle
ssecond
480 nanometers
Energy =
Frequency =
Wavelength =
Bohr also knew Balmer’s equation for determining the wavelength of the light waves that are emitted from hydrogen gas.
42
The Bohr Atom - Evolution of Electron Configuration
Model
Obse
rvati
ons
Theory
Bohr Puts the Pieces Back Together
22
21 n
1
n
1
nm 91
11
Bohr did this by using Balmer’s equation to see if that equation will give an answer of 480 nanometers.
Lets try using the Balmer equation to find a light’s wavelength for two different situations.
(1) Wavelength when n = 3 and n = 2.
2 1
(2) Wavelength when n = 4 and n = 2.
2 1
43
The Bohr Atom - Evolution of Electron Configuration
= =
== )( 5 3691
1[ ]-1
)(0.14911[ ]
-1
0.0015[ ]-1
670 nanometers
= )( -3
1 2
1
91
12 2[ ]
-1
)( -n3
1 n2
112 291[ ]
-1
=
= ?
)(911 (9 -
4)(4 9)
[ ]-1
Bohr Puts the Pieces Back TogetherM
odel
Obse
rvati
ons
Theory
Bohr model expects a transition of an electron from orbit 4 (n=4) back to orbit 2 (n=2) to have a wavelength of 480 nanometers. Balmer’s equation says that a transition from n=3 to n=2 produces a light with a wavelength equal to 670 nanometers.
(1) Wavelength when n = 3 and n = 2.
2 1
44
The Bohr Atom - Evolution of Electron Configuration
=)( 12 6491
1[ ]-1
)(0.19911[ ]
-1
480 nanometers
= )( -4
1 2
1
91
12 2[ ]
-1
)( -n4
1 n2
112 291[ ]
-1
=
= =0.0021[ ]-1
== ? )(911 (16 -
4)(4 16)[ ]-1
Model
Obse
rvati
ons
Theory
Do you think the answer is going to be greater or less than 670 nanometers this time?
Why?
(2) Wavelength when n = 4 and n = 2.
2 1
Bohr Puts the Pieces Back Together
45
The Bohr Atom - Evolution of Electron Configuration
Model
Obse
rvati
ons
Theory
Bohr atomic model (1913) expects a transition of an electron from orbit 4 back to orbit 2 to have a wavelength of 480 nanometers.
Bolmer’s equation developed in 1885 says;
=670 nanometers )( -3
1 2
112 291[ ]
-1
=
(1) Wavelength when n = 3 and n = 2.
2 1
=480 nanometers )( -4
1 2
112 291[ ]
-1
=
(2) Wavelength when n = 4 and n = 2.
2 1
Bohr’s atomic model works for the hydrogen atom.
Bohr Puts the Pieces Back Together
46
The Bohr Atom - Evolution of Electron Configuration
Electrons only change orbits if specific amounts (quanta) of extra energy from the outside world are involved. Electrons that receive enough extra energy from the outside world can leave the atom they are in.
Electrons that return to orbits they used to reside in give up the extra energy they acquired when they moved in the first place.
Electronic energy given up when electrons move back into an original orbit often show up as a specific color light.
Electrons that leave one orbit must move to another orbit.
Summary of Bohr’s (Planetary) Model (1913)
Electrons orbit the nucleus in different orbits at different fixed distances.
47
The Bohr Atom - Evolution of Electron Configuration
Good news!
Bohr’s model explains the light waves that are emitted by hydrogen!
Bad News!
Bohr’s model does not explains the light waves that are emitted by any other atom!
Good news!The expanded quantum theory does explain light waves that are emitted by other atoms!
48
The Bohr Atom - Evolution of Electron Configuration
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