MCAT Physics: Atomic and Nuclear Phenomena

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Photoelectric Effect
– When light of a sufficiently high frequency is incident on a metal in a vacuum, the metal ions emit electrons
– liberated electrons produce a current
Threshold Frequency
Threshold Frequency
– The minimum frequency of light that causes ejection of electrons (ft)
– photoelectric effect is an all or nothing response
Photon
– light quanta in a light beam
Energy of each photon
E=hf
– energy of each photon is proportional to light
h= 6.626 x 10^-34
– energy of a photon increases with increasing frequency
Maximum kinetic energy of the ejected electron
Maximum kinetic energy of the ejected electron
where W is the work function of the metal in question
– energy can be anywhere from 0 to Kmax
Work Function
– The minimum energy required to emit an electron
– W=h ft (ft = threshold frequency)
– activation energy in a sense
Absorption and Emission
– when an electron falls from a higher energy level to a lower energy level, a photon of light is emitted with an energy equal to the energy difference between the two orbitals
UV- Vis Spectroscopy
– looks at the absorption of light in the visible and ultraviolet range
Fluorescence
– if you excite a fluorescent substance with UV radiation it will begin to glow with visible light
Mass Defect
– mass of the nucleus is NOT the sum of the masses of all the protons and neutrons inside of it
– The actual mass is slightly smaller due to matter being converted to energy
E=mc^2
– because c is squared, a small amount of mass will yield a huge amount of energy
Strong Nuclear Force
– When protons and neutrons come together to form the nucleus they are attracted to each other by this force
– binding energy allows the nucleons to bind together
Weak Nuclear Force
– much less strong than strong nuclear force
Isotopic notation
Isotopic notation
Fusion
Fusion
– small nuclei combine to form a larger nucleus
Fission
– large nucleus splits into smaller nuclei
– chain reaction
– powers most commercial power plants
Radioactive Decay
– naturally occurring spontaneous decay accompanied by the emission of specific particles

Know these types of problems
1) the integer arithmetic of particle and isotope species
2) Radioactive half life problems
3) The use of exponential decay curves and decay constants

Isotope Decay Arithmetic and Nucleon Conversion
– the sum of the atomic numbers must be the same on both sides of the equation and the sum of the mass numbers must be the same on both sides as well
Alpha Decay
Alpha Decay
– Emission of an alpha particle
– alpha particle consists of two protons, two neutrons and zero electrons
– atomic number is 2 less and mass number is 4 less
Beta Decay
– emission of a beta particle which is an electron
– emitted when a neutron decays into a proton, a beta particle and an antineutrino
Beta - Decay
Beta – Decay
– neutron converted to a proton and a beta – particle
– atomic number is one higher and mass number does not change
Beta + Decay
– a proton is converted into a neutron and a beta + particle
– the atomic number will be one lower and the mass number will not change
Gamma Decay
Gamma Decay
– The emission of gamma rays which are high energy photons
– carry now charge and simply lower the energy of the parent nucleus without changing the mass or atomic number
Electron Capture
Electron Capture
– capture an inner electron that combines with a proton to form a neutron
– atomic number is one less but the mass number stays the same
Half Life
Half Life
– the time it takes for half of the sample to decay
Exponential Decay
Exponential Decay
– the rate at which nuclei decay is proportional to the number that remain
– gamma is decay constant
Exponential Decay
Exponential Decay
– n(o) is the number of undecayed nuclei at t=0
Decay constant and half life
Decay constant and half life
ln 2=0.693
Categories: Atomic Physics