EAS100 Lecture Notes - Lecture 5: Uranium-235, Radiometric Dating, Rubidium

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EAS 100 Lecture 5 Chemistry in Earth Sciences
Atoms
An atom consists of a nucleus surrounded by electrons. The nucleus has positively charged
protons and neutral neutrons.
Important values:
Number of protons = atomic number on periodic table
Total protons + neutrons = atomic mass
Many elements have various isotopes the same element with the same number of protons,
but with a varying number of neutrons. These isotopes may be unstable (radioactive).
Ex: Carbon is usually stable in it’s most common form Carbon 12. An isotope is Carbon
14 which is radioactive and can be used for radiometric dating.
Radioactive Decay
By looking at how long it takes for half of a radioactive parent element to decay, we can
accurately pinpoint how old an object is. This is very useful because radioactive decay is a
nuclear process, which means it remains unaffected by external factors! (heat, pressure, etc.
will NOT change the rate of radioactive decay!)
Can also be called Isotopic Dating, since we often see radioactive isotopes decay into a
nonradioactive isotope of another element. We know how old something is by looking at these
couplings, the ratio of the parent isotope to the other, and the exact half-life.
Common examples:
Carbon 14 to Nitrogen 14 has a half life of 5 730 years
Uranium 235 to Lead 207 has a half life of 710 million years
Rubidium 87 to Strontium 87 has a half life of 50 billion years.
Atoms and Ions
An atom can be considered neutral if the amount of positively charged protons equals the
amount of negatively charged electrons. If there is an excess of either positive or negative
charge, it is called an Ion.
Cation excess positive charge
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Document Summary

Eas 100 lecture 5 chemistry in earth sciences. An atom consists of a nucleus surrounded by electrons. The nucleus has positively charged protons and neutral neutrons. Important values: number of protons = atomic number on periodic table, total protons + neutrons = atomic mass. Many elements have various isotopes the same element with the same number of protons, but with a varying number of neutrons. Ex: carbon is usually stable in it"s most common form carbon 12. 14 which is radioactive and can be used for radiometric dating. By looking at how long it takes for half of a radioactive parent element to decay, we can accurately pinpoint how old an object is. This is very useful because radioactive decay is a nuclear process, which means it remains unaffected by external factors! (heat, pressure, etc. will not change the rate of radioactive decay!)

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