Ch9: Atomic and Nuclear Phenomena

Photoelectric Effect

e- ejection from metal surface from high frequency light

Current

Net charge flow/time from e-

Increase light intensity = Increase current

Threshold Frequency (fT)

Min light frequency causing e- ejection

Depend on chemical composition of material/metal

f < fT

No e- ejected

Not enough photon energy

f > fT

e- ejected

Max Ek = hf - hfT (work function)

Photons

Light quanta

Frequency vs Wavelength

Increase frequency = Decrease wavelength = Increase photon energy

Electron Kinetic Energy

Excess energy above Ft converted to e- kinetic energy

Increase light energy = Increase atom electrical potential energy = Increase e- kinetic energy

Maximum Electron Kinetic Energy

All photon energy transferred to e-

Work Function (W)

Min energy to eject e-

Depend on chemical composition of material/metal

Particle Theory of Light

Light contains discrete energy bundles not a continuous wave

Supported by photoelectric effect

Photoelectric Effect: Change Light Colour

Change speed of e- ejected

Photoelectric Effect: Change Light Intensity

Change number of e- ejected

Bohr Model

Stable and discrete e- levels (orbits)

Bohr Model: Photon Absorption

e- jump from low-energy to high-energy orbit

Same frequency as energy diff between orbits

Bohr Model: Photon Emission

e- fall from high-energy to low-energy orbit

Same frequency as energy diff between orbits

Absorption Spectra

Impacted by small molecular structure changes

Infrared (IR) Spectroscopy

Determine chemical structure of compounds

Diff bonds absorb diff light wavelengths

UV-Vis Spectroscopy

Determine visible and UV light absorption

Indicators

Diff absorption patterns based on protonation state

Usually have conjugated double bonds or aromatic rings

Fluorescence

Exciting fluorescent substance with UV radiation

Excited e- return to ground state in 2+ steps emitting photons in process

Photon energy > Fluorescence radiation

Mass Defect

Nuclei mass smaller than protons + neutrons

Unbonded nucleon mass - bonded nucleon mass

Mass converted to energy in nuclear fusion

Nucleons

Protons and neutrons

Strong Nuclear Force

Attraction between protons and neutrons to form nucleus

Strongest force

Act over small distances

Binding Energy

Diff in energy between bonded system and unbonded constituents energy levels

Radiated away (heat, light, EM radiation)

Nucleus Stability: Binding Energy

Iron: Stable nucleus, peak binding energy

Intermediate-sized nuclei: More stable than large or small

Weak Nuclear Force

Small contribution to nucleus stability

4 Fundamental Forces of Nature

Strong nuclear force

Weak nuclear force

Electrostatic forces

Gravitation

Isotopic Notation

Fusion

Small nuclei combine into larger nucleus

Release energy

Fission

Large nucleus split into smaller nuclei

Rarely spontaneous

Release energy

Induced Fission

Fission reactions releasing more neutrons cause chain reaction of fission in nearby atoms

Radioactive Decay

Natural spontaneous nuclei decay emitting particles

Nucleon Conservation

Balanced nuclear decay reactions

Atomic number sum and mass number sum same on both sides

Alpha Decay

Emit a-particle

Daughter nucleus A’ = A-4 and Z’ = Z-2

a-Particle

4/2 He nucleus

2 protons, 2 neutrons, 0 electrons

Charge: +2

Very large

Non-penetrating

Beta-Negative Decay

Emit b(-) particle and antineutrino

Daughter nucleus A’ = A and Z’ = Z+1

b-Particle

e-

Charge: -1

Very small

More penetrating

Beta-Positive Decay (Positron Emission)

Emit positron (b+) and neutrino

Daughter nucleus A’ = A and Z’ = Z-1

Positron (e+)

e- mass with + charge

Gamma Decay

Emit y-rays

Detect on atomic absorption spectrum

Daughter nucleus A’ = A and Z’ = Z

y-Rays

High energy/frequency photons

No charge

Lower parent nucleus energy

Electron Capture

Absorb inner e- to combine with proton

Form neutron and release neutrino

Daughter nucleus A’ = A and Z’ = Z-1

Half-Life (T 1/2)

Time for half of sample to decay

Remaining amount → 0

Half-Life Common Q: Given half-life and time passed

time passed/half-life = half-lives passed

(1/2)^half-lives passed = amount remaining

Exponential Decay

Rate of radioactive nuclei decay proportional to number of nuclei remaining