Evolution of Electron Configuration The Bohr Atom Evolution of Electron Configuration 2

Observations Model 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. Observations Model > Develop a story (theory) to predict what will happen in the next experiment. > Find equations (i.e., models) that explain and support the theory. Make into an animation Theory This repetitive process continues as the knowledge base increases.

Aristotle’s “Composition of the Universe” Theory (354 BC) Learning about the Electron 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. Dalton’s “Atomic” Theory (1808) Tiny indivisible particles that have no sub-particle sized parts. J. J. Thomson’s “Plum Pudding” Model (1898) Sub-particle sized parts exist in an atom but no idea of their arrangement. Ernest Rutherford’s “Nucleus” Model (1909) Electrons are independent sub-particle sized parts occupying their own specific space.

Summary of Bohr’s (Planetary) Model (1913) Learning about the Electron Summary of Bohr’s (Planetary) Model (1913) Electrons orbit the nucleus in different orbits at different fixed distances. Electrons that leave one orbit must move to another orbit. 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.

Observations Theory Model The Puzzle Pieces Bohr had to Solve The most important observation Bohr needed to explain was the fact that when energy was added to atoms the atoms gave off (emitted) light. Theory The biggest problem with Rutherford’s story was why the electrons don’t eventually crash into the nucleus and destroy the atom. Model Up to this point in time (1913), nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms. Make into an animation A difficult puzzle to solve, but Bohr made a major step in putting the pieces together.

When an element is subjected to heat The Puzzle Pieces Bohr had to Solve Observations Observations 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 When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light. 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. Make into an animation As the temperature increases from 750 degrees, more yellow and blue light is emitted and mixes with the red light to give white light.

The Puzzle Pieces Bohr had to Solve Observations Observations 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 Light Source Photons Photoelectric Effect Albert Einstein (1905) (Nobel prize 1921) Make into an animation Light (photons) can force electrons to be emitted from the surface of a metal.

As described in “The Tempest” What/Where The Puzzle Pieces Bohr had to Solve Observations Observations 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. Corona Who, What, Where is it? Who St. Elmo’s Fire Make into an animation As described in “The Tempest” What/Where

Observations Corona (1611) The Puzzle Pieces Bohr had to Solve Observations Observations When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light. Corona (1611) “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 divide And burn in many places; on the topmast, The yards and bowsprit, would I flame distinctly, Then meet and join.” Make into an animation A passage from “The Tempest” by William Shakespeare

Observations Bohr’s postulates include the belief that The Puzzle Pieces Bohr had to Solve Observations Observations 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 3 (n = 3) electrons are in orbits but the orbits are at different distances from the nucleus. Orbit number 1 (n = 1) Ernest Rutherford’s Model (1909) Make into an animation electrons that leave one orbit must move to another orbit.

The Puzzle Pieces Bohr had to Solve Observations Observations When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light. Bohr’s concept of a carbon atom with a “ground” state electron configuration. 2 electrons in orbit number 3 2 electrons in orbit number 1 2 electrons in orbit number 2 C 6 12 remember an atom that has 6 protons is always called a carbon atom Make into an animation Atom with 6 protons and 6 electrons “Ground” state electron configuration

C Observations 12 6 The Puzzle Pieces Bohr had to Solve When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light. The electron configuration of an atom changes when energy (heat, light, or electricity) is added to the atom. electrons only change orbits if specific amounts (quanta) of extra energy from the outside world are involved. Make into an animation C 6 12 Atom with 6 protons and 6 electrons “Ground” state electron configuration “Excited” state electron configuration

C Observations 12 6 The Puzzle Pieces Bohr had to Solve When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light. Energy is removed from the atom when an electron returns to its original orbit. Make into an animation C 6 12 Atom with 6 protons and 6 electrons “Excited” state electron configuration “Ground” state electron configuration

C Observations 12 6 The Puzzle Pieces Bohr had to Solve add energy this electron used to be in orbit #2 C 6 12 add energy “Excited” state electron configuration “Higher” energy state for an atom Make into an animation “Ground” state electron configuration “Lower” energy state for an atom electrons only change orbits if specific amounts (quanta) of extra energy from the outside world are involved.

C Observations 12 6 The Puzzle Pieces Bohr had to Solve release energy this electron has returned to orbit #2 C 6 12 release energy “Excited” state electron configuration Make into an animation “Lower” energy state for an atom “Ground” 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. electrons only change orbits if specific amounts (quanta) of extra energy from the outside world are involved.

The Puzzle Pieces Bohr had to Solve Observations Observations 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. When an element is subjected to heat, “Ground” state electron configuration Light Source Photons Photoelectric Effect N 7 14 Make into an animation 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.

When an element is subjected to heat, light, The Puzzle Pieces Bohr had to Solve Observations Observations 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, When an element is subjected to heat, light, or electrical discharge, positive electrode negative electrode 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. Make into an animation Corona and Plasma Discharges

Observations The Puzzle Pieces Bohr had to Solve When an element is subjected to heat, light, or electrical discharge, it can, under certain conditions, give off light. positive electrode negative electrode Make into an animation Helium corona and plasma discharge

Theory Bohr’s postulates include the belief that Model Observations Theory The Puzzle Pieces Bohr had to Solve Theory Theory The biggest problem with Rutherford’s story was why the electrons do not eventually crash into the nucleus and destroy the atom. Bohr’s postulates include the belief that Electrons can only have a specific energy value to be in a specific orbit of an atom. Electrons remain in a specific orbit unless they obtain additional energy to move into orbits that are further from the nucleus. Make into an animation Therefore, electrons are not satellites (Rutherford’s theory) like man made satellites in continuous decaying orbits that gradually spiral toward the center.

Theory The Puzzle Pieces Bohr had to Solve 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 Make into an animation (This is a tiny amount of energy, but remember that an atom is a tiny thing in the first place.)

Theory The Puzzle Pieces Bohr had to Solve 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. Orbit number Energy electron needs 2.18 x 10 -18 Joules 1 0.55 x 10 -18 Joules Make into an animation 2 0.24 x 10 -18 Joules 3

Bohr’s Energy Level Equation Postulate Model Observations Theory The Puzzle Pieces Bohr had to Solve Model Model Up to this point in time, nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms. Bohr’s Energy Level Equation Postulate The energy of an electron orbit is inversely related to the square of the orbit number. The energy of an electron orbit is inversely related to the square of the orbit number. 2 2.18 x 10 -18 (orbit number) electron energy level equals Joules Make into an animation 2.18 x 10 -18 2 (n) = Joules E

Model = = = The Puzzle Pieces Bohr had to Solve Up to this point in time, nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms. Bohr’s Energy Level Equation Postulate The Bohr model for calculating the energy relative to an atomic orbital. E Joules 2.18 x 10 -18 2 (n) = n The Bohr model calculated energy for orbit the closest to nucleus. Make into an animation n =1 2.18 x 10 -18 (n) 2.18 x 10 -18 Joules (1) = E 1 = =

Model = The Puzzle Pieces Bohr had to Solve Up to this point in time, nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms. The Bohr model for calculating the energy relative to an atomic orbital. E Joules 2.18 x 10 -18 2 (n) = n The Bohr model calculated energy for second (n=2) orbit. Make into an animation n =2 2.18 x 10 -18 0.55 x 10 -18 Joules 2.18 x 10 4 = E 2 = = (2) (2)

Model = The Puzzle Pieces Bohr had to Solve Up to this point in time, nobody knew why Balmer’s equation predicted the color (frequency) of the light waves emitted by atoms. The Bohr model for calculating the energy relative to an atomic orbital. E Joules 2.18 x 10 -18 2 (n) = n The Bohr model calculated energy for third (n=3) orbit. Make into an animation n =3 2.18 x 10 -18 2.18 x 10 -18 9 0.24 x 10 -18 Joules = E 3 = = (3) (3)

Model The Puzzle Pieces Bohr had to Solve The Bohr model energy calculation for the: second (n=2) orbit, E 2 = 2.18 x 10 -18 (2) 0.55 x 10 Joules 4 third (n=3) orbit, E 3 = 2.18 x 10 -18 (3) 9 0.24 x 10 Joules Make into an animation and the fourth (n=4) orbit. 2.18 x 10 -18 16 0.14 x 10 -18 Joules = E 4 =

= E2 – E4 Model The Puzzle Pieces Bohr had to Solve The Bohr model energy calculation for second (n=2) orbit. E 2 = 2.18 x 10 -18 (2) 0.55 x 10 Joules 4 0.55 x 10 -18 2.18 x 10 -18 16 0.14 x 10 -18 Joules = 0.14 x 10 -18 fourth (n=4) orbit E 4 = energy difference for electron transitions between second and fourth orbits. Make into an animation = E2 – E4 0.55 x 10 -18 Joules 0.55 x 10 -18 0.14 x 10 -18 0.14 x 10 -18 Joules = -

Model [ ] The Puzzle Pieces Bohr had to Solve -18 -18 0.55 x 10 - energy difference for electron transitions between second and fourth orbits. -18 -18 [ ] = 0.55 x 10 - 0.14 x 10 Joules 0.41 x 10 -18 energy difference = Joules 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. Make into an animation

How do you think he did that? The Puzzle Pieces Bohr had to Solve Model Model Niels Bohr believed that Joules 0.41 x 10 -18 of energy would 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. Make into an animation How do you think he did that?

f Model h = = energy released Planck’s constant The Puzzle Pieces Bohr had to Solve Model Model Niels Bohr believed that Joules 0.41 x 10 -18 of energy would be released when an electron went from orbit 4 to orbit 2 in an atom. (1) Bohr knew he could use Max Planck’s equation to connect the energy of a light wave to its frequency (color). h Joules 0.41 x 10 -18 1 Planck’s constant energy released f photon = = 2 3 4

f f Model = h = The Puzzle Pieces Bohr had to Solve Niels Bohr believed that Joules 0.41 x 10 -18 of energy would be released when an electron went from orbit 4 to orbit 2 in an atom. Joules 0.41 x 10 -18 f photon = h Joules 0.41 x 10 -18 Joules/second 6.63 x 10 -34 f photon 1 = 2 3 4

f f Model = = The Puzzle Pieces Bohr had to Solve Joules 0.41 x 10 -18 Joules · second 6.63 x 10 -34 f photon = -18 6.63 +34 x 10 0.41 cycles second f photon = 1 2 3 4

f f f Model = = = The Puzzle Pieces Bohr had to Solve 6.63 x 10 0.41 -18 6.63 +34 x 10 0.41 cycles second f photon = +16 cycles second 0.41 6.63 x 10 = f photon 0.062 x 10 +16 cycles second f photon = 1 2 3 4

f f Model = = The Puzzle Pieces Bohr had to Solve cycles 0.062 x 10 +16 cycles second f photon = 0.062 x 10 +14 cycles second f photon = 6.2 x 10 1 2 3 4

Do you remember that equation? The Puzzle Pieces Bohr had to Solve Model Model 0.41 x 10 -18 Niels Bohr now believed that the Joules of energy released 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 +14 cycles second 6.2 x 10 Niels Bohr also knew the relationship between the frequency of a light wave and its wavelength. 1 2 3 Do you remember that equation? 4

l Model = The Puzzle Pieces Bohr had to Solve 0.41 x 10 -18 Niels Bohr believed that Joules of energy is the energy associated with a light wave that has a frequency of +14 cycles second 6.2 x 10 Shortest distance between the dotted lines is the wavelength of this wave. length of a wave speed of light frequency of light wave l = 1 2 3 4

l l Model = = The Puzzle Pieces Bohr had to Solve 0.41 x 10 -18 Niels Bohr believed that Joules of energy is the energy associated with a light wave that has a frequency of +14 cycles second 6.2 x 10 Shortest distance between the dotted lines is the wavelength of this wave. length of a wave speed of light l = +14 cycles second 6.2 x 10 1 +8 meters 3.0 x 10 second 2 l = 3 6.2 x 10 +14 cycles second 4

l l l l Model = = = = The Puzzle Pieces Bohr had to Solve 6.2 x 10 +14 cycles second +8 meters 3.0 x 10 l = Shortest distance between the dotted lines is the wavelength of this wave. length of a wave l 6.2 -14 +8 3.0 x 10 cycle meters x 10 = 6.2 -6 3.0 x 10 cycle meters l = 1 2 l -6 0.48 x 10 cycle meters = 3 4

l l l Model = = = The Puzzle Pieces Bohr had to Solve 0.48 x 10 -6 Shortest distance between the dotted lines is the wavelength of this wave. length of a wave l cycle meters = 0.48 x 10 l -9 cycle meters = 480 x 10 1 2 l = 480 nanometers 3 4

l Energy Frequency 480 nanometers Wavelength Model The Puzzle Pieces Bohr had to Solve Model 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: Energy Frequency Wavelength 480 nanometers Joules 0.41 x 10 -18 6.2 x 10 +14 cycles second l

l Model The Puzzle Pieces Bohr had to Solve Joules 0.41 x 10 -18 Energy = 6.2 x 10 +14 cycles second Frequency = Wavelength = 480 nanometers Bohr also knew Balmer’s equation for determining the wavelength of the light waves that are emitted from hydrogen gas. l

÷ ø ö ç è æ - = l n nm 91 Bohr Puts the Pieces Back Together Model Observations Theory Bohr Puts the Pieces Back Together Bohr did this by using Balmer’s equation to see if that equation will give an answer of 480 nanometers. ÷ ø ö ç è æ - = l 2 1 n nm 91 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

[ ] [ ] [ ] [ ] [ ] ) ( ) ( ) ( ) ( l l l = - 3 1 2 91 n3 n2 91 1 Model Observations Theory Bohr Puts the Pieces Back Together (1) Wavelength when n = 3 and n = 2. 2 1 = ) ( - 3 1 2 91 [ ] -1 l n3 n2 ) ( 91 1 (9 -4) (4 9) [ ] -1 ) ( 5 36 91 1 [ ] -1 ) ( 0.14 91 1 [ ] -1 l = ? = = 0.0015 [ ] -1 l = 670 nanometers = 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.

[ ] [ ] [ ] [ ] [ ] ) ( ) ( ) ( ) ( l l l = - 4 1 2 91 n4 n2 Why? = Model Observations Theory Bohr Puts the Pieces Back Together (2) Wavelength when n = 4 and n = 2. 2 1 = ) ( - 4 1 2 91 [ ] -1 l n4 n2 Do you think the answer is going to be greater or less than 670 nanometers this time? Why? = = ? l ) ( 91 1 (16 -4) (4 16) [ ] -1 = ) ( 12 64 91 1 [ ] -1 ) ( 0.19 91 1 [ ] -1 = 0.0021 [ ] -1 l 480 nanometers

[ ] [ ] ) ( ) ( l l = 670 nanometers - 3 1 2 91 = 480 nanometers - 4 1 Model Observations Theory Bohr Puts the Pieces Back Together Bolmer’s equation developed in 1885 says; = 670 nanometers l ) ( - 3 1 2 91 [ ] -1 (1) Wavelength when n = 3 and n = 2. = 480 nanometers l ) ( - 4 1 2 91 [ ] -1 (2) Wavelength when n = 4 and n = 2. Bohr atomic model (1913) expects a transition of an electron from orbit 4 back to orbit 2 to have a wavelength of 480 nanometers. Bohr’s atomic model works for the hydrogen atom.

Summary of Bohr’s (Planetary) Model (1913) Electrons orbit the nucleus in different orbits at different fixed distances. Electrons that leave one orbit must move to another orbit. 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.

Good news! Bad News! 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!