- ATOMIC STRUCTURE:
Atoms are made of Protons (positively charged), Neutrons (neutral or no charge), and electrons (negatively charged).
The protons and neutrons are close together in the central part, called the nucleus.
The electrons move more freely, and sometimes go away and come back during chemical reactions.
Atoms of the same element exist in different varieties, called isotopes.
Isotopes of the same element have the same number of protons but different numbers of neutrons (and thus different masses).
Isotopes are designated by superscripts preceding the abbreviation for the element, so 1H, 2H, and 3H are different isotopes of hydrogen
and 12C and 14C are two isotopes of carbon.
Heavy elements like lead and uranium have many isotopes.
- RADIOACTIVITY:
Many isotopes are stable, but some isotopes are unstable and undergo change or "decay". Several kinds of decay are known. The two most common are:
Alpha-decay means loss of a Helium nucleus
Beta-decay means loss of a high-speed electrons
Radioactive decay happens at a predictable rate, called a "half-life" (H or t1/2 )
If you start with a quantity N0 then the equation for radioactive decay is:
N = N0 (1/2)T/H
where N0 is the initial quantity (at the start)
N is the quantity that remains
T is the elapsed time
H is the half-life
Thus, after one half-life, half of the original quantity remains;
after two half-lives, 1/4 of the original quantity remains, equal to (1/2)2
after 3 half-lives, 1/8 of the original quantity remains, equal to (1/2)3
after 4 half-lives, 1/16 of the original quantity remains, equal to (1/2)4
This process is sometimes called exponential decay because time is in the exponent of the equation.
- RADIOACTIVE DATING
The half-life of a radioactive element is constant. It does not vary with temperature or any other environmental condition.
Scientists can therefore carefully measure the amount of radioactive decay that has taken place and calculate the elapsed time.
Different isotopes decay at different rates. Some are so unstable that their half-life is measured in tiny fractions of a second.
Other isotopes have half-lives measured in minutes, hours, or years.
Earth scientists use isotopes with very long half-lives. Among the most useful isotopes are these:
- 14C Half-life = 5,730 years
(decays into Nitrogen)
- 40K Half-life = 1.28 billion years
(decays into Argon)
- 238U Half-life = 4.5 billion years
(decays ultimately into Lead))
- 87Rb Half-life = 49.2 billion years
(decays into Strontium)
Thus, carbon-14 dating is used to date archaeological remains of the past few thousands of years,
while potassium-argon dating is used for time periods measured in millions of years, and dating using
uranium or rubidium decay is mostly used for the oldest rocks.
- ABSOLUTE DATING
Radioactive dating is a form of Absolute dating because dates are determined numerically, in years or millions of years.
If absolute dates can be determined for a few geological formations, then correlation can be used to date
other rocks of the same age, and absolute dates can be inserted into the geological time column.
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