Chapter 19: Radioactivity & Nuclear Chemistry
I)
Discovery of Radioactivity
1. Radioactivity is the emission of subatomic particles of high-energy
electromagnetic radiation by the nuclei of certain atoms. Such atoms are said
to be radioactive.
2. Radioactivity was first discovered by Henri Becquerel in 1896. Becquerel
was very interested in X-rays (recently discovered by the German physicist
Roentgen). He performed studies of phosphorescent uranium containing
compounds on X-ray photographic plates. He discovered that the compound
constantly emitted a type of emission which he termed uranic rays.
3. Soon after Becquerel’s discovery, Marie & Pierre Curie discovered that
other elements besides uranium were radioactive. Marie Curie discovered the
elements polonium & radium; and is credited for coming up with the name
radioactivity. She was awarded two Nobel Prizes (physics & chemistry).
II)
Types of Radioactvity (see Figures 19.1 - 19.4 & Table 19.1)
1. Rutherford and other scientists began to investigate the findings of
Becquerel & the Curies. They discovered that the radioactive emissions were
coming from the nuclei of radioactive atoms.
2. Review isotope notation, atomic number, mass number from chapter 2. In
nuclear chemistry, the nucleus of an individual isotope is called a nuclide.
3. Know the main sub-atomic particles and their notations (see pg. 868).
4. Natural radioactivity can be divided into several different types.
A) Alpha decay
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-
Characterized by having same identity as helium-4 nucleus.
The most massive of radioactive particles; it has high ionizing
power but very low penetrating power. Alpha particles easily
stopped using a sheet of paper.
Original atom (parent nuclide) loses 2 protons and 2 neutrons to
form new atom (daughter nuclide).
Mass number decreases by 4 and atomic number decreases by 2.
Use nuclear equations to express how decay occurs
Equation must balance in terms of mass and atomic
numbers.
2
B) Beta decay
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Particle emitted is an electron.
Electron formed when neutron changes into a proton.
Mass number remains fixed; atomic number increases by 1.
Less massive than alpha particle; it has lower ionizing power but
higher penetrating power. Need a metal sheet or wooden block to
stop this guy.
C) Gamma Ray Emission
-
Gamma radiation is a type of electromagnetic radiation. Gamma
rays are very high energy photons.
Gamma emission does not change either mass or atomic numbers
Lowest ionizing power; highest penetrating power. Need thick
lead shielding or thick slabs of concrete to stop these rays.
D) Positron Emission
-
When an unstable nucleus emits a positron (the antiparticle of an
electron).
Positrons have the same mass as an electron, but are oppositely
charged.
Mass number remains fixed; atomic number decreases by 1.
Ionizing & Penetrating powers similar to beta particles.
E) Electron Capture
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III)
This one is different in that a particle is absorbed by an unstable
nucleus (all the others involve a particle being ejected from a
nucleus).
Electron capture occurs when a nucleus assimilates an electron
from an inner orbital of its electron cloud. Positrons have the
same mass as an electron, but are oppositely charged.
Mass number remains fixed; atomic number decreases by 1.
Kinetics of Radioactive Decay: Radiometric Dating
1. Radioactivity is a natural component of our environment. The soil, air,
food and water all contain background radiation.
2. Every element with 83 or more protons in its nucleus is radioactive.
3. All radioactive nuclei decay via first-order kinetics (see chapter 13); so the
rate of decay takes the form
Rate = kN
3
4. The time it takes for one-half of the parent nuclides in radioactive sample
to decay to the daughter nuclides is called the half-life. It is identical to
the half-life concept covered for chemical reactions in chapter 13.
5. To calculate the half-life, we use the same equation as before.
t1/2 = 0.693 / k
6. The integrated rate law for these problems follows the same format as
found in chapter 13 (here Nt is number of radioactive nuclei at time t and
N0 is the initial number of nuclei)
ln (Nt / N0) = - kt
IV)
Nuclear Fission & Nuclear Fusion
1. The dawn of the “atomic or nuclear” age happened in the 1930s through
the efforts of several scientists (Fermi, Meitner, Hahn, etc.).
2. Two terms that are routinely used, they are fission and fusion.
3. Nuclear fission involves the splitting of a heavier atom to yield a lighter
nuclei and one or more particles. This is how a nuclear reactor plant
generates electricity.
4. Nuclear fusion is the merging of two light nuclei to form a heavier one.
The most common example of this is the Sun (a giant fusion chamber).
Hydrogen bombs also use this process.
5. Although fusion provides about 10X the amount of energy per gram of
fuel (versus fission), it is not yet practical to use this approach mainly due
to the high temperature requirement.