PCAT: Nuclear Chemistry

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binding energy
– the amount of energy required to break up a given nucleus into its constituent protons and neutrons
– that energy is converted to mass via E = mc^2, resulting in a larger mass for the constituent protons and neutrons that that of the original nucleus; this difference is called the mass defect
– peaks at iron — implies that iron is the most stable atom
– intermediate nuclei are most stable that large and small nuclei
mass defect
– every nucleus (besides H) has a smaller mass than the combined mass of its constituent protons and neutrons — the difference of this is the mass defect
– is a result of matter that has been converted to energy
chemical reactions
– atoms can be rearranged by the formation or breaking of chemical bonds
– reactions generally result in the release or absorption of small amounts or energy
– reaction rates are generally affected by catalysts, temperature, or pressure
– only electrons in the affected orbital of the atom are involved in the formation and breaking of bonds
nuclear reactions
– elements or isotopes are changed from one to another
– reactions result in the release or absorption of large amounts of energy
– reaction rates are generally not affected by catalysts, temperature, or pressure
– protons, neutrons, or electrons can be involved
nuclear reaction examples
– fusion
– fission
– radioactive decay
– involved either combining or splitting the nuclei of atoms
fusions
– occurs when small nuclei combine into a larger nucleus
– eg. stars like the sun power themselves by fusing 4 hydrogen nuclei to make one helium nucleus producing 4 x 10^26 J every second
– can only take place at extremely high temperatures, generally referred to as thermonuclear reactions
fission
– a process in which a large, heavy (mass number > 200) atom splits to form smaller, more stable nuclei (especially noble gases) and one or more neutrons
– because the original large nucleus is more unstable than its products, there is the release of a large amount or energy
– spontaneous fission rarely occurs — by the absorption of a low-energy neutron, fission can be induced in certain nuclei
– scientists use fission reactions to power commercial nuclear electric-generating plants
radioactive decay
– a naturally occurring spontaneous decay of certain nuclei accompanies by the emission of specific particles
– can be classified as a certain type of fission
– radioactive decay problems are or three general types:
1. the integer arithmetic or particle and isotope species
2. radioactive half-life problems
3. the use of exponential decay curves and decay constants
alpha decay
– the emission of an alpha particle which is a 4 2 He nucleus that consists of two protons and two neutrons
– the alpha particle is very massive (compared to a beta particle) and double charged (since it contains 2 protons and has a +2 charge)
– alpha particles interact with matter very easily — they do not penetrate shielding (such as lead sheets) very far
– the emission of an alpha particle means that the daughter’s atomic number (Z’) will be less than the parents atomic number, and the daughters mass number (A’) will be 4 less than the parents mass number:
Zdaughter = Zparent – 2
Adaughter = Aparent – 4
alpha decay and fission
are the only radioactive decay processes on the exam during which the mass number (A’) changes
beta decay
– the emission of a ß-particle, which could either be ß- (electron) or ß+ (positron) from the nucleus
– beta radiation from radioactive decay is more penetrating than alpha radiation
ß- decay
– a neutron decays into a proton and a ß- particle (and an antineutrino)
– parents mass number is unchanged
– parents atomic number is increased by 1
Zdaughter = Zparent + 1
Adaughter = Aparent
ß+ decay
– a proton decays into a neutron and a ß+ particle (and a neutrino)
– parents mass number is unchanged
– parents atomic number is decreased by 1
Zdaughter = Zparent – 1
Adaughter = Aparent
positron
e+
– similar to an electron (has minimal mass), but has a positive charge
– do not normally reside in the nucleus but is emitter when a proton or neutron in the nucleus decays
gamma decay
– the emission of gamma particles which are high energy photons
– usually follows another type of nuclear decay
– a way for the nucleus to shed excess energy (similar to how an electron in an excited state emits a photon to shed energy)
– gamma particles carry no charge and simply lower the energy of the emitting (parent) nucleus without changing the mass number or the atomic number
electron capture
– certain unstable radionuclides are capable of capturing an inner electron that combines with a proton to form a neutron
– the atomic number is now one less than the original
– the mass number remains the same
– rare process best thought of as an inverse ß- decay, following the exact same process and beta minus decay but in reverse
radioactive decay half-life
T1/2
– the time is takes for half of the sample to decay by any of the above processes
exponential decay
– let N be the number of radioactive nuclei that have not yet decayed in a sample
– the rate at which the nuclei decay (∆N / ∆t) is proportional to the number that remain (N):
∆N / ∆t = -gamma N
– where gamma is the decay constant
– the solution to the above equation tells us how the number of radioactive nuclei changes with time (exponential decay):
N = N0 e^-gamma(t)
– where N0 is the number of undecayed nuclei at t = 0
decay constant
gamma = ln (2) / (T1/2) = 0.693 / (T1/2)
Categories: Nuclear Chemistry