Nuclear physics unravels the mysteries of atomic nuclei and radioactivity. We'll explore the structure of nuclei, including protons and neutrons, and how they're held together by the strong nuclear force. Understanding nuclear stability and binding energy sets the stage for grasping radioactive decay.
Radioactive decay comes in different flavors: alpha, beta, and gamma. We'll learn how these processes change atomic nuclei and emit particles or energy. and decay rates help us predict radioactive behavior, while decay series show us how unstable nuclei transform over time.
Atomic Structure
Components of the Atomic Nucleus
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Atomic nucleus forms the dense central core of an atom containing most of its mass
Protons carry positive electric charge and contribute to the of an element
Neutrons possess no electric charge but add to the of an atom
Isotopes represent atoms of the same element with different numbers of neutrons
Affect the atomic mass while maintaining the same chemical properties
Can be stable or unstable (radioactive)
Strong nuclear force binds protons and neutrons together within the nucleus
Overcomes the electrostatic repulsion between positively charged protons
Nucleus size ranges from ~1-10 femtometers (1 fm = 10^-15 m)
Significantly smaller than the overall atom (10^-10 m)
Nuclear Stability and Binding Energy
Nuclear stability depends on the ratio of protons to neutrons
Stable nuclei generally have roughly equal numbers of protons and neutrons
Binding energy represents the energy required to break apart a nucleus into its constituent nucleons
Calculated using Einstein's mass-energy equivalence formula: E=mc2
Nuclear binding energy per nucleon peaks around iron (Fe-56)
Explains why fusion and fission reactions release energy
Mass defect refers to the difference between the mass of a nucleus and the sum of its constituent nucleon masses
Directly related to binding energy through E=mc2
Radioactive Decay
Types of Radioactive Decay
Radioactivity involves the spontaneous emission of particles or energy from unstable atomic nuclei
occurs when a nucleus emits an alpha particle (two protons and two neutrons)
Reduces the atomic number by 2 and the mass number by 4
Common in heavy nuclei (uranium, thorium)
happens when a converts to a or vice versa
Beta minus (β^-) decay emits an and an antineutrino
Beta plus (β^+) decay emits a positron and a neutrino
Changes the atomic number by 1 while keeping the mass number constant
Gamma radiation involves the emission of high-energy photons
Often accompanies alpha or beta decay as nuclei transition to lower energy states
Does not change the atomic number or mass number
Nuclear Equations and Conservation Laws
Nuclear equations represent radioactive decay processes using atomic symbols and particle notation
92238U→90234Th+24He (alpha decay)
614C→714N+e−+νˉe (beta minus decay)
Conservation laws apply to nuclear reactions
Conservation of electric charge
Conservation of nucleon number (mass number)
Conservation of energy (including rest mass energy)
Decay energy (Q-value) calculated from mass difference between parent and daughter nuclei
Determines the kinetic energy of emitted particles and radiation
Half-Life and Decay Series
Half-Life and Radioactive Decay Rates
Half-life represents the time required for half of a radioactive sample to decay
Characteristic property of each radioisotope
Ranges from fractions of a second to billions of years
(λ) relates to half-life through the equation: t1/2=λln(2)
Exponential decay law describes the number of undecayed nuclei over time
N(t)=N0e−λt, where N_0 is the initial number of nuclei
Activity (A) measures the rate of radioactive decays per unit time
A=λN, often expressed in becquerels (Bq) or curies (Ci)
dating uses the known half-life of ^14C (5,730 years) to determine the age of organic materials
Effective for dating objects up to about 50,000 years old
Decay Series and Radioactive Equilibrium
Decay series describe the sequential decay of radioactive nuclei through multiple steps
End with a stable nuclide (often an isotope of lead)
Three naturally occurring decay series
Uranium series (starts with ^238U)
Thorium series (starts with ^232Th)
Actinium series (starts with ^235U)
Branching occurs when a nucleus can decay through multiple pathways
Probabilities of different decay modes sum to 100%
Secular equilibrium established in decay chains when parent half-life greatly exceeds that of daughters
Activity of each member in the chain becomes equal
Important in geological dating and understanding natural radioactivity