Physical SciencePhysical ScienceAtomic PhysicsAtomic Physics

Slides subject to changeSlides subject to change

Modern PhysicsModern Physics Focus on the atomic, microscopic scale.Focus on the atomic, microscopic scale. Properties of electrons, and the electron Properties of electrons, and the electron

cloud around each nucleus.cloud around each nucleus. Properties of the nucleus, Properties of the nucleus,

subatomic particles.subatomic particles.

Nature of light.Nature of light.

Also QuantizationAlso Quantization At the level of atoms, many quantities are At the level of atoms, many quantities are

found only in integer multiples of found only in integer multiples of elementary amounts (for example, elementary amounts (for example, electric electric charge echarge e).).

These microscopic quantities are said to These microscopic quantities are said to be be quantizedquantized − it’s as tiny as you can get, − it’s as tiny as you can get, cannot be subdivided.cannot be subdivided.

PhotonsPhotons

In 1905 Einstein proposed that In 1905 Einstein proposed that light exists as tiny packets, or particles, which he called photons. Light is Light is quantizedquantized..

“Can be absorbed or generated only as a whole."

We live in a quantum universe, one built out of tiny, discrete chunks of energy and matter.

Light EnergyLight Energy A photon has no mass, yet it has energy.A photon has no mass, yet it has energy. Energy of one photon equals Planck’s Energy of one photon equals Planck’s

constant constant hh times the frequency times the frequency ff of the light of the light wave. wave.

E = hfE = hf

Planck’s constant” Planck’s constant” hh = 6.626x10 = 6.626x10–34–34 J-s J-s

Green LaserGreen Laser Energy of Energy of oneone photon from the green laser: photon from the green laser:

ff = 5.64x10 = 5.64x101414 Hz Hz

EEphotonphoton= = hfhf = (6.626x10= (6.626x10–34–34)(5.64x10)(5.64x101414)) = 3.74x10= 3.74x10–19–19 J J

Photons Interact with AtomsPhotons Interact with Atoms When light is absorbed by an object, the When light is absorbed by an object, the

absorption occurs absorption occurs in the atomsin the atoms of the of the object. Emission of light comes from the object. Emission of light comes from the atomsatoms..

AbsorptionAbsorption

EmissionEmission

Things That Are InvisibleThings That Are Invisible Hebrews 11:3 Hebrews 11:3 By faith we understand that By faith we understand that

the universe was formed at God's the universe was formed at God's command, so that what is seen was not command, so that what is seen was not made out of what was visible. made out of what was visible.

Dalton Model 1807Dalton Model 1807 An atom is ...An atom is ...

tinytiny indivisibleindivisible uniformly denseuniformly dense solid spheresolid sphere

Each chemical element is composed of Each chemical element is composed of tiny, indivisible particles called tiny, indivisible particles called atomsatoms, , which are identical for that element.which are identical for that element.

Billiard Ball ModelBilliard Ball Model

J.J. Thompson ModelJ.J. Thompson Model Electron discovered in 1897 by English Electron discovered in 1897 by English

physicist J.J. Thompson. physicist J.J. Thompson. But atoms are electrically neutral, so he But atoms are electrically neutral, so he

proposed the negative charges are spread proposed the negative charges are spread throughout the atom (1903).throughout the atom (1903).

Perhaps electrons are like raisins in Perhaps electrons are like raisins in Christmas “plum pudding?”Christmas “plum pudding?”

Thompson’s “Plum Thompson’s “Plum Pudding” ModelPudding” Model

Ernest Rutherford ModelErnest Rutherford Model Ernest Rutherford (New Zealand chemist) Ernest Rutherford (New Zealand chemist)

discovers atoms have small charged nucleus discovers atoms have small charged nucleus (1911).(1911).

His students Ernest Marden and Hans Geiger aim His students Ernest Marden and Hans Geiger aim alpha particles (alpha particles (helium nucleihelium nuclei) at thin gold foil.) at thin gold foil.

1 out of every 8,000 particles were reflected.1 out of every 8,000 particles were reflected.

Rutherford’s ModelRutherford’s Model All the positive charge is crammed inside a All the positive charge is crammed inside a

tiny, massive nucleus. tiny, massive nucleus. Nucleus is equivalent to a marble in the Nucleus is equivalent to a marble in the

middle of a football stadium. middle of a football stadium.

Rutherford’s Rutherford’s “Planetary” Model“Planetary” Model

The much lighter electrons are The much lighter electrons are well outside the nucleus.well outside the nucleus.

Atoms are almostly entirely Atoms are almostly entirely empty space.empty space.

Nuclear DimensionsNuclear Dimensions

Hydrogen EmissionHydrogen Emission When light is emitted from a pure gas When light is emitted from a pure gas

(say, in a gas discharge tube) there are a (say, in a gas discharge tube) there are a limited number of frequencies in the limited number of frequencies in the line line spectrumspectrum. Why?. Why?

Bohr ModelBohr Model

Niels Bohr model (1913) looks like a planetary model.Niels Bohr model (1913) looks like a planetary model. Orbit radius, and total energy of each electron orbit Orbit radius, and total energy of each electron orbit

has a has a quantumquantum value. value. Result: electrons only orbit at certain Result: electrons only orbit at certain radiusesradiuses. . It has lowest energy when it is closest to the nucleus, It has lowest energy when it is closest to the nucleus,

higher energy farther from the nucleus.higher energy farther from the nucleus.

Bohr Model for HydrogenBohr Model for Hydrogen An electron’s energy in its orbit is An electron’s energy in its orbit is

characterized by a whole number value, characterized by a whole number value, principal quantum numberprincipal quantum number nn = 1, 2, 3 ... = 1, 2, 3 ...

When hydrogen electron is When hydrogen electron is in the lowest state (closest in the lowest state (closest to the nucleus),to the nucleus), n n = 1, it’s in = 1, it’s in the the ground stateground state..

Bohr ModelBohr Model Hydrogen Hydrogen emitsemits a a photonphoton when the electron when the electron

drops from a higher energy level to a lower drops from a higher energy level to a lower energy level.energy level.

EmissionEmission

Bohr ModelBohr Model Hydrogen Hydrogen absorbsabsorbs a a photonphoton when when E=hfE=hf is is

exactly the right energy to enable a jump from exactly the right energy to enable a jump from a lower energy level to a higher energy level.a lower energy level to a higher energy level.

AbsorptionAbsorption

Bohr CharacteristicsBohr Characteristics Bohr hydrogen orbit radii Bohr hydrogen orbit radii rr11 = “Ground State” radius = “Ground State” radius

= 0.053x10= 0.053x10−9−9 m = 0.053 nanometer. m = 0.053 nanometer. rr22 = 4 r = 4 r11

rr33 = 9 r = 9 r11

rrnn = n = n22 r r11

Energy in OrbitEnergy in Orbit Bohr hydrogen orbit energy Bohr hydrogen orbit energy EE11 = “Ground State” energy = “Ground State” energy

= − 13.6 electron volts (approximately = − 13.6 electron volts (approximately 2.18x102.18x10−18−18 joules. joules.

EE22 = E = E11/4 = −3.4 eV/4 = −3.4 eV EE33 = E = E11/9 = −1.5 eV/9 = −1.5 eV

EEnn = E = E11/n/n22

HydrogenHydrogen

––13.613.6

––3.43.4

––1.51.5

EE11

EE22

EE33

Sometimes a Sometimes a hydrogen hydrogen electron’s electron’s energy is drawn energy is drawn like this:like this:

When atom absorbs energy, electron

goes to a higher

energy level.

1 electron volt (eV) is 1.6x101 electron volt (eV) is 1.6x10–19 –19 JJ

Ele

ctro

n vo

lts (e

V)

Ele

ctro

n vo

lts (e

V)

Ground State at this energy levelGround State at this energy level

HydrogenHydrogen Electron needs energy to jump to higher state. Electron needs energy to jump to higher state.

May come from heat (bumped by another May come from heat (bumped by another atom), light (photons), or electricity (bumped atom), light (photons), or electricity (bumped by electrons).by electrons).

Enables it to jump from Enables it to jump from EE11 to say, to say, EE22 or higher. or higher.

Energy must be exact amount.Energy must be exact amount.

HydrogenHydrogen Electrons give up energy to jump to a Electrons give up energy to jump to a

lower state. lower state. May jump from May jump from EE22 down to down to EE11.. Will give up exact amount with a photon of Will give up exact amount with a photon of

E = hfE = hf..

––13.613.6

––3.43.4

––1.51.5

EE11

EE22

EE33

Ground State at this energy levelGround State at this energy level

Sometimes a hydrogen Sometimes a hydrogen electron’s energy is drawn electron’s energy is drawn

like thislike this

When atom When atom emits emits

energy, energy, electron electron goes to goes to

lower level.lower level.

Ele

ctro

n vo

lts (e

V)

Ele

ctro

n vo

lts (e

V)

1 electron volt (eV) is 1.6x101 electron volt (eV) is 1.6x10–19 –19 JJ

Energy TransitionsEnergy Transitions If an H atom is in excited state EIf an H atom is in excited state E22, it will normally , it will normally

spontaneously emit a photon to go from Espontaneously emit a photon to go from E22 →→ E E11.. ΔΔEE = = EE11 – – EE22 = (= (−− 13.6) 13.6) −− ( (−−3.4)3.4)

= –10.2 eV ·1.6x10= –10.2 eV ·1.6x10−19 −19 J/1 eVJ/1 eV= –1.63x10= –1.63x10–18–18 J J

The minus sign indicates the atom lost energy The minus sign indicates the atom lost energy (the energy goes into a photon).(the energy goes into a photon).

Energy TransitionsEnergy Transitions From Einstein and Bohr,From Einstein and Bohr, loss of energy goes into loss of energy goes into

a emission of a photon of specific frequency.a emission of a photon of specific frequency. E = hfE = hf = 1.63x10 = 1.63x10–18–18 J J

ff = 1.63x10 = 1.63x10–18–18 / 6.634x10 / 6.634x10–34–34

ff22→→11 = 2.46x10= 2.46x101515 Hz Hz

This is a photon in the ultraviolet range.This is a photon in the ultraviolet range.

Energy StatesEnergy States The characteristic red line in the hydrogen The characteristic red line in the hydrogen

spectrum represents an electron transition spectrum represents an electron transition from the from the nn = 3 state to = 3 state to nn = 2 state. = 2 state.

More on Light In 1905 Einstein proved that energy and matter

are linked in the most famous relationship in physics: E = mc2.  

The energy content of a body is equal to the mass of the body times the speed of light squared.

He suggested that the heat produced by radium could mark the conversion of tiny amounts of the mass of the radium salts into energy.

Even More on Light Mass can be converted into energy and energy

into mass. Example, there are two particles with the same

mass, an electron and a positron (an “anti-particle”). Their mass is each 9.11x10−31 kg. Total equivalent energy is 2mec2 = 1.64x10−13

J. Equal but opposite charge −e, +e.

Even More on Light What is a photon of that energy? E = hf 1.64x10−13 = (6.626x10−34) f f = 2.47x1020 Hz, a gamma ray.

An electron-positron collision conserves total charge, and annilates both particles and creates a gamma ray.

Historical PerspectivesHistorical Perspectives Up to 1900, “Classical Physics” Up to 1900, “Classical Physics”

Laws of motion Laws of motion Galileo, NewtonGalileo, Newton Gravitation Gravitation NewtonNewton Electricity Electricity CoulombCoulomb Magnetism, Magnetism, FaradayFaraday, , OrstedOrsted Electromagnetic waves Electromagnetic waves MaxwellMaxwell OpticsOptics

Post-1900, “Modern Physics” Post-1900, “Modern Physics”

LasersLasers LLight ight AAmplification by mplification by SStimulated timulated EEmission of mission of

RRadiation (LASER).adiation (LASER). Some atoms can stay in certain “metastable” energy Some atoms can stay in certain “metastable” energy

states for a long period of time.states for a long period of time. Sending a photon through the material will “stimulate” Sending a photon through the material will “stimulate”

transition from metastable state to a lower statetransition from metastable state to a lower state Creates an avalanche of photons that bounce off Creates an avalanche of photons that bounce off

mirrors at each end.mirrors at each end. Keep pumping energy into Keep pumping energy into lasinglasing medium. medium.

Heisenberg UncertaintyHeisenberg Uncertainty At the microscopic level, it is impossible to At the microscopic level, it is impossible to

know a particle’s exact position and know a particle’s exact position and velocity simultaneously.velocity simultaneously.

There is a limit on measurement precision There is a limit on measurement precision at microscopic levels.at microscopic levels.

The measurement process The measurement process itself disturbs what you are itself disturbs what you are measuring in this measuring in this environment.environment.

Absorption and EmissionAbsorption and Emission When visible light of all wavelengths pass When visible light of all wavelengths pass

through a cool sample of the gas, we get through a cool sample of the gas, we get an an absorptionabsorption spectrum. spectrum.

High voltage across a sample, get an High voltage across a sample, get an emissionemission spectrum. spectrum.

AbsorptionAbsorption

EmissionEmission

Electron CloudElectron Cloud Erwin SchrErwin Schröödinger model (1926).dinger model (1926). The electron’s position is is only a The electron’s position is is only a

probability. probability.

HydrogenHydrogen

Ground state, Ground state, most probable most probable

radius ~0.053 nmradius ~0.053 nm

Exactly the same Exactly the same as Bohr’s model.as Bohr’s model.

Matter WavesMatter Waves If light can be a wave and act like a particle, can particles act like waves?If light can be a wave and act like a particle, can particles act like waves? Louis deBroglie proposed: Louis deBroglie proposed:

λλparticleparticle = = hh//mvmv

Example: Wavelength of small ball, 0.5 kg at 26 m/sExample: Wavelength of small ball, 0.5 kg at 26 m/sλλparticleparticle = = hh//mv = (6.634x10mv = (6.634x10−34−34)/(0.5)(26) = 5.1x10)/(0.5)(26) = 5.1x10−35−35 m m

Too short to see any effect.Too short to see any effect.

Electron DiffractionElectron Diffraction Electron beam through a crystal Electron beam through a crystal doesdoes

show diffraction effects.show diffraction effects.

Light, As BulletsLight, As Bullets

Double SlitDouble Slit

You expect light to hit the target as shown.You expect light to hit the target as shown.

However, …However, …

Split PersonalitySplit Personality The particles form a diffraction pattern.The particles form a diffraction pattern.

Double SlitDouble Slit

CCD camera CCD camera detects single detects single

photonsphotons

Diffraction Diffraction patternpattern