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Understanding Our Planet Through Chemistry

Elements, Isotopes, and Radioactivity

Matter is made up of atoms, and atoms are made up of a complex array of subatomic particles. Let's consider only three of these particles: protons (positively charged), neutrons (no charge), and electrons (negatively charged). Every element has a fixed number of protons that cannot be changed without creating a different element. If, for example, we add a proton to an atom of sulfur, it becomes heavier and is now an atom of chlorine. If we change the number of neutrons in an atom, however, it has almost no effect on the chemical properties and outward appearance but does have an effect on the atomic mass. It can also have an extreme effect on the atomic stability of the element. If we take an atom of potassium that has 39 neutrons in it and add one more, the atom now becomes unstable and can radioactively decay.

Each combination of an element with a different number of neutrons is called an isotope. Isotopes that are radioactive disintegrate or decay in a predictable way and at a specific rate to make other isotopes. The radioactive isotope is called the parent, and the isotope formed by the decay is called the daughter. A radioactive isotope decays at a constant rate proportional to the number of radioactive atoms remaining. A simple way of describing the speed of decay is to see the time it takes for half of the atoms of a radioactive parent to decay and form the daughter element(s). This is called the half life. Various events (especially melting of the rock) will cause the isotopes in a rock to redistribute. When the rock solidifies it can be thought of as starting a stopwatch. By determining the amount of the parent and daughter isotopes present scientists can determine when the stopwatch started.

Schematic showing parent-daughter isotope decaying process.Naturally occuring radioactive isotopes (called the parent isotope) disintegrate at specific rates to make other isotopes (called daughter isotopes). The amount of time it takes for half the quantity of the original isotope to decay is constant, no matter how much is present at the beginning. Based on this principle, the age of geologic events can be measured. [47k]

As an example, the parent-daughter system used to determine the age of the Earth is the uranium-lead system. The decay of the parent uranium isotopes to daughter lead isotopes in samples of the Earth, Moon, and meteorites indicates that all the planets in our solar system formed 4.5 billion years ago.

While determining the age of the Earth is intriguing, radiometric dating has recently been useful in more practical issues like the following: With what age of granite formation are ore deposits in a particular region associated? How recently has a fault been active, and is it likely to be safe to build near it now? How often does a volcano erupt and how often do landslides recur?

Photo of Mt. St. Helen's 1980 eruption. On May 18, 1980 the Cascade volcano, Mt. St. Helens, erupted exposively causing a great deal of destruction and a number of deaths. [106k]

Photo of Washington state city with volcano in background.In addition to telling us the Earth is 4.5 billion years old, geologic dating can answer important questions such as: When was the last time a fault moved? What areas are a safe place to construct a nuclear reactor? How frequently does a particular volcano erupt? Is a volcano nearing an eruptive part of its cycle? [112k]

Because different isotopes of an element have different masses, they can be viewed as an arrangement of masses in a spectrum. An instrument that separates and electronically measures a spectra of atomic masses is called a mass spectrometer. There are many types of mass spectrometers, but the most frequently used in earth-science age determinations are magnetic sector mass spectrometers. These magnetic spectrometers operate on the principle that if you put an electric charge on an object and throw it into a magnetic field, the object's path will form a circle. The radius of the circle will depend on the strength of the magnetic field and the mass of the charged atom divided by its electric charge. Thus, if you have a purified portion of an element from a sample with several isotopes, each can be made, in sequence, to travel the same circular path to the detector by varying the strength of the magnetic field. Magnetic sector mass spectrometers consist of at least three components as illustrated in this figure. (1) A source of sample ions, (2) a magnetic field, and (3) a detector.

Schematic drawing illustrating atoms undergoing mass spectrometry analyses.The atoms on the filament are ionized and accelerated at a specific velocity through a magnetic field, causing them to take a specific curved path depending on the ion's mass. This type of mass spectrometer scheme most commonly used in geologic dating shows how ions with a specific mass are directed into the collector for counting, while others, like a race car taking the curves at the wrong speed, are lost. [18k]

Photo of sample filament next to a sewing needle eye for size comparison. A difficult chemical procedure is used to concentrate the element of interest so that isotopes can be measured on a mass spectrometer. In many cases the recovered amount is no larger than a spot on the sample filament and could pass through the eye of a needle. [140k]

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