Radioactivity in geologic dating

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Scientists discovered that rocks could be timepieces -- literally.

Many chemical elements in rock exist in a number of slightly different forms, known as isotopes.

So, for example, every carbon atom contains six protons and six electrons, but the number of neutrons in each nucleus can be six, seven, or even eight.

Therefore, carbon has three isotopes, which are specified as carbon-12, carbon-13 and carbon-14 (figure 1). Comparison of stable and unstable atoms of the element carbon.

But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.

But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.

However, while the number of neutrons varies, every atom of any chemical element always has the same number of protons and electrons.

The age of the planet, though, was important to Charles Darwin and other evolutionary theorists: The biological evidence they were collecting showed that nature needed vastly more time than previously thought to sculpt the world.

A breakthrough came with the discovery of radioactivity at the beginning of the 1900s.

Certain isotopes are unstable and undergo a process of radioactive decay, slowly and steadily transforming, molecule by molecule, into a different isotope.

This rate of decay is constant for a given isotope, and the time it takes for one-half of a particular isotope to decay is its radioactive half-life.

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