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Chapter 19

CHEM 105bL Chapter Notes - Chapter 19: Beta Decay, Alpha Decay, Electron Capture


Department
Chemistry
Course Code
CHEM 105bL
Professor
Parr Jessica
Chapter
19

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Chapter XIX: Radioactivity and Nuclear Chemistry
I. Types of Radioactivity
- Natural radioactivity can be categorized into: alpha (α) decay, beta (β) decay, gamma (γ)
ray emission, and positron emission.
- IN unstable atomic nuclei, it is possible to gain stability by absorbing an e- from one of
the atom’s own orbitals in electron capture
-
- A is the mass number (the sum of protons and neutrons in the nucleus); Z is the atomic
number (the number of protons in the nucleus); X is the chemical symbol
- The number of neutrons in the nucleus (N) = A – Z
- A nuclide is the particular isotope or species of an element
- Subatomic particles are expressed as such: (proton), (neutron), and (electron)
-Alpha (α) decay occurs when an unstable nucleus emits a particle composed of 2
protons and 2 neutrons ()
- Alpha radiation has the highest ionizing power and have the most potential to interact
with other molecules and atom by ionizing them
- Alpha particles are the largest in size and have the lowest penetrating power
-Beta (β) decay occurs when an unstable nucleus emits an electron ()
- It can be represented with this nuclear eqn:
- When an atom emits a beta particle, its atomic # increases by one because there is now
an additional proton
- Beta radiation has a lower ionizing power than alpha decay and a higher penetrating
power
-Gamma (γ) ray radiation is electromagnetic radiation and has no charge or mas, it is
emitted with all types of radiation ()
- Gamma rays have the lowest ionizing power but the highest penetrating power
-Positron emission occurs when an unstable nucleus emits a positron, the antiparticle of
an e- (they have identical mass but opposite charge) ()
- Positron emission can be represented with this nuclear eqn:
- The atomic number decreases by 1 because there is one less proton after emission
-Electron capture occurs when a nucleus assimilates an e- from an inner orbital of its e-
cloud (it converts a proton into a neutron)
- Electron capture can be represented with the nuclear eqn:
II. The Valley of Stability: Predicting the Type of Radioactivity
- The binding of a nucleus is provided by a fundamental force of physic known as the
strong force
- All nucleons (protons and neutrons) are attracted to each other by the strong force
- Nuclear stability relates strongly to the ration of neutrons to protons (N/Z)
- When N/Z is too high, they tend to convert neutrons protons via beta decay
- When N/Z is too low, they tend to convert protons to neutrons via positron emission or
electron capture (sometimes alpha decay)
III. The Kinetics of Radioactive Decay and Radiometric Dating
- All radioactive nuclei decay via first-order kinetics, the rate of decay is directly
proportional to the number of nuclei present (Rate=kN where k is the rate constant and N is
the number of radioactive nuclei)
- The rate of decay varies among nuclides
- The half-life is the amount of time it takes for ½ of the parent nuclides to decay into
daughter nuclides ()
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