RADIOACTIVE DECAY AND HEATING
Some atomic nuclei are not stable. If they are allowed to sit,
they will spontaneously break apart into smaller nuclei releasing
particles and energy. For example, the
isotope Uranium 238 is
unstable in that it will spontaneously break apart into
Thorum 234
and a Helium with the release of some energy (see the panel on the
right) with a half-life of 4.5 billion years.
Uranium 238
and Thorium 234 are sometimes written in shorthand as at right.
For the Uranium 238, the 92 says that its nucleus contains 92 protons and
the 238 says that its nucleus is made of 92 protons and 146 neutrons.
For the Thorium, the 90 says that its nucleus contains 90 protons and
the 234 says that its nucleus contains 90 protons and 144 neutrons.
Here, we will stick to saying that Uranium 238 decays to Thorium 234
and a Helium nucleus plus energy for clarity.
This decay has a half-life of
4.5 billion years. The meaning of
half-life is that it tells you
how long it takes half of the uranium 238 in a box to decay
into thorium 234. This property of radioactive decay lends
itself well to the age dating of rocks and other materials.
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Some important short half-life radioactive nuclei are
Aluminium 26 decays to Magnesium 26 with a half-life of 720,000 years
Iodine 129 decays to Xenon 129
with a half-life of 16,000,000 years
Some important long half-life radioactive nuclei are
Potassium 40 decays to Argon 40
with half-life 1,250,000,000 years
Thorium 232 decays to lead 208
with half-life 14,000,000,000 years
Uranium 235 decays to lead 207
with half-life 700,000,000 years
Uranium 238 decays to lead 206
with half-life 4,500,000,000 years
The shorter half-life nuclei add to the initial heating of the planets
while the longer half-life nuclei add to the current heating of the
interiors of the planets (the interiors of some of the Terrestrial
planets are still quite hot. Currently, Potassium 40
is the most important
radioactive heat source for the Earth.