The Bohr Atom Evolution of Electron Configuration

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The Bohr Atom - Evolution of Electron Configuration

The Bohr Atom Evolution of Electron Configuration

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Model

ObservationsTheory

Learning about the ElectronTechnicians, 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.

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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

http://www.sciencemuseum.org.uk/on-line/electron/section2/biography.html

http://www.rutherford.org.nz/biography.htm

http://antoine.frostburg.edu/chem/senese/101/atoms/dalton.shtml

http://www.utm.edu/research/iep/d/democrit.htm

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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

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The Bohr Atom - Evolution of Electron ConfigurationThe Puzzle Pieces Bohr had to Solve M

odel

Obse

rvat

ions

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 ConfigurationThe Puzzle Pieces Bohr had to Solve

Obse

rvat

ions

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.


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Obse

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ions

Observations

Light Source

Photons

Photoelectric Effect Albert 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. Photoelectric Effect

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Obse

rvat

ions

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”


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Obse

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ions

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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Obse

rvat

ions


Bohr’s postulates include the belief thatOrbit 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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Obse

rvat

ions

2 electrons in orbit number 1


Observations

C 6

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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



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Obse

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ions

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.


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The Bohr Atom - Evolution of Electron ConfigurationThe Puzzle Pieces Bohr had to SolveObservations

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


Obse

rvat

ions

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Obse

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ions

Observations


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

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Obse

rvat

ions

Observations


“Excited” state electron configuration“Higher” energy state for an

atomWhen 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


this electron has returned to orbit #2C

6

12

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Obse

rvat

ions

Observations


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

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Obse

rvat

ions

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.

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Obse

rvat

ions

positive electrode

negative electrodeHelium corona and plasma discharge

The Puzzle Pieces Bohr had to SolveObservations


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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

rvat

ions

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

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The Bohr Atom - Evolution of Electron ConfigurationThe Puzzle Pieces Bohr had to SolveTheory

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.)

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The Bohr Atom - Evolution of Electron ConfigurationThe Puzzle Pieces Bohr had to SolveTheory

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.

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The Bohr Atom - Evolution of Electron ConfigurationThe Puzzle Pieces Bohr had to SolveModel

Up to this point in time, nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms.

Model

Obse

rvat

ions

Theory

Model

Bohr’s Energy Level Equation Postulate

electron energy level

equals Joules2

2.18 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.

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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.

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Model

E Joules

2.18 x 10

-18

2(n)=n


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=

=


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Model

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

=

=



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Model

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

=

=

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Model

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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Model

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.

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Model

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?

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Model


Joules

0.41 x 10

-18of energywould be released when an electron went from

orbit 4 to orbit 2 in an atom.

1234

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

hJoules

0.41 x 10

-18

http://www-groups.dcs.st-andrews.ac.uk/~history/Mathematicians/Planck.html

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Model

1234

=fphoton

Joules

0.41 x 10

-18

h

=fphoton

Joules

0.41 x 10

-18

Joules/second

6.63 x 10

-34


Joules

0.41 x 10

-18of energywould be released when an electron went from

orbit 4 to orbit 2 in an atom.

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Model

1234

=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

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Model

1234

=fphoton

fphoton

= 0.062 x 10+16 cycle

ssecond

+16 cyclessecond

0.416.63

x 10

=fphoton

cyclessecond

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6.63

+34x 10

0.41

x 10

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Model

1234

fphoton

= 0.062 x 10+16 cycle

ssecond

fphoton

= 6.2 x 10

+14 cyclessecond

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Model

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

1234

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?

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Model

1234

Shortest distance between the dotted lines is the wavelength of this wave.

length of a wave

= speed of lightfrequency of light wave


Joules

0.41 x 10

-18of energyis the energy associated with a

light wave that has a frequency of

6.2 x 10+14 cyclessecond

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Model

1234

= speed of light6.2 x 10

+14 cyclessecond

=6.2 x 10+14 cycle

ssecond

+8 meters3.0 x 10 second


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

secondShortest distance between the dotted lines is the wavelength of this wave.

length of a wave

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Model

1234

=

=


+8 meters3.0 x 10 second

6.2

-14+83.0 x 10cycle

metersx 10

Shortest distance between the dotted lines is the wavelength of this wave.

length of a wave

=6.2

-63.0 x 10cycle

meters

= -60.48 x 10cycle

meters

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Model

1234

=

=

cyclemeters

480 nanometers

480 x 10-9

= -60.48 x 10cycle

metersShortest distance between the dotted lines is the wavelength of this wave.

length of a wave

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Model

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

Wavelength 480 nanometers


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Model

Joules

0.41 x 10

-18


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.

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Model

Obse

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Theory

Bohr Puts the Pieces Back Together

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21 n

1 n1

nm 9111

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


http://www-history.mcs.st-andrews.ac.uk/history/Mathematicians/Balmer.html

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= =

== )( 5 3691

1[ ]-1

)(0.14911[ ]-1

0.0015[ ] -1670 nanometers

= )( -31

21

911

2 2[ ]-1

)( - n3

1 n2

112 291[ ]-1

=

= ?

)(911 (9 -

4)(4 9)[ ]-1

Bohr Puts the Pieces Back TogetherM

odel

Obse

rvat

ions

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.


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=)( 12 6491

1[ ]-1

)(0.19911[ ]-1

480 nanometers

= )( -41

21

911

2 2[ ]-1

)( - n4

1 n2

112 291[ ]-1

=

= =0.0021[ ] -1

== ? )(911 (16 -

4)(4 16)[ ]-1

Model

Obse

rvat

ions

Theory

Do you think the answer is going to be greater or less than 670 nanometers this time?

Why?



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Model

Obse

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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 )( - 31

211

2 291[ ]-1

=


=480 nanometers )( - 41

211

2 291[ ]-1

=


Bohr’s atomic model works for the hydrogen atom.


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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.

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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!

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The Bohr Atom Evolution of Electron Configuration

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