When we hear the word “isotope,” most of us would think of radioactivity. Radioactive analysis is the most commonly known form of isotopic analysis, but it is only a part.
Radioactive dating relies on known half-lives of different isotopes, but not all isotopes are radioactive. Nonradioactive isotopes do not change into other elements and so do not have half-lives, but certain ones can be quite useful nonetheless.
Isotopes are atoms of the same element that have different atomic masses, or extra neutrons in the nucleus.
Chemistry depends on the interactions of protons with electrons, so on the first order the chemical properties of the isotopes are nearly the same.
The key word in understanding nonradioactive isotopic analysis and its usefulness to scientists is “nearly.”
In first-order reactions, all isotopes are the same, but second-order reactions preferentially use lighter isotopes in favor of heavier ones.
Oxygen is a good example.
Three naturally occurring isotopes of oxygen all contain eight protons in the nucleus and eight electrons in differing energetic orbits surrounding the nucleus. The eight electrons react with other atoms to determine the first-order properties. The three isotopes have eight, nine and 10 neutrons in the nucleus.
All atoms are moving: vibrating, spinning or rotating. At a given temperature the atoms in a substance such as oxygen gas have an equal distribution of energies. In atomic terms: Energy is motion.
This microscopic energy is in the form of kinetic energy, which depends on the mass and the square of the velocity. For a given energy, atoms with greater mass must be moving at a lower speed, which makes them less reactive.
Although this might come across as arcane, it is a kinetic process that is important in any chemistry involving oxygen.
For example, single-celled foraminifera take calcium and carbonate ions out of seawater to build their reef structure. Carbonate ions consist of one atom of carbon and three atoms of oxygen.
Foraminifera will preferentially extract the lighter isotope of oxygen, with eight neutrons in the nucleus, over the one with nine neutrons in the nucleus because the lighter isotope is moving faster and is thus more reactive.
In foraminifera the ratio of oxygen-18 to oxygen-16 is different from in seawater by small amounts, but enough to measure in the chemistry lab. By itself this is not particularly useful, but we find that the ratio of oxygen-18 to oxygen-16 used by the foraminifera changes with temperature.
When we analyze sediments that contain foraminifera, the ratio of oxygen-18 to oxygen-16 can reveal the temperature of near-surface seawater at the time the foraminifera were alive in the water.
Isotopes were in the news a few years back when analyses of material from flybys of comets and asteroids revealed that protium (hydrogen-1) to deuterium (hydrogen-2) ratios of Earth’s water are closer to those of asteroids rather than comets.
This fact puts serious dents in the theory that Earth’s water came from collisions with comets during its formation.
Of equal or greater importance in understanding Earth’s history are polar ice cores where the measurement is relatively simple: Less-heavy oxygen in the frozen water means that temperatures were cooler.
These are only three examples of the use of isotopes in geochemistry. There are many others in geochemistry and biochemistry involving nonradioactive isotopes.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
