In some elements, the number of neutrons in the atomic nucleus may vary. Different isotopes of the same element have the same number of protons but different numbers of neutrons, as you saw in the story opening this chapter. Many elements have several isotopes. Generally, isotopes are formed when atoms combine and/or release particles (decay). The isotopes of hydrogen shown here have special names, but for most elements their isotopes do not have special names.

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The natural isotopes of carbon, for example, are ¹²C (six neutrons in the nucleus), ¹³C (seven neutrons), and ¹⁴C (eight neutrons). Note that all three (called carbon-12, carbon-13, and carbon-14) have six protons, so they are all carbon. Most carbon atoms are ¹²C, about 1.1 percent are ¹³C, and a tiny fraction are ¹⁴C. The carbon atoms that make up complex biological molecules are mostly ¹²C, but some are ¹³C. The ratio of ¹³C:¹²C varies with location and can be used to identify biological samples whose origin is not known (Investigating Life: Determining Beef Source in Big Macs Using Isotope Analysis). All carbon isotopes have virtually the same chemical reactivity, which is an important property for their use in experimental biology and medicine.

An element’s atomic weight (or relative atomic mass) is equivalent to the average of the mass numbers of a representative sample of atoms of that element, with all the isotopes in their normally occurring proportions. More precisely, an element’s atomic weight is defined as the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of ¹²C. Because it is a ratio, atomic weight is a dimensionless physical quantity—it is not expressed in units. The atomic weight of hydrogen, taking into account all of its isotopes and their typical abundances, is 1.00794. This number is fractional because it is the average of the contributing masses of all of the isotopes. This definition implies that in any given sample of hydrogen atoms of a particular element found on Earth, the average composition of isotopes will be constant. But as you saw in the opening to this chapter, that is not necessarily so. Some water has more of the heavy O isotopes. So chemists are now listing atomic weights as ranges, for example, H: 1.00784–1.00811.

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Most isotopes are stable. However, some, called radioisotopes, are unstable and spontaneously give off energy in the form of α (alpha), β (beta), or γ (gamma) radiation from the atomic nucleus. Known as radioactive decay, this release of energy transforms the original atom. The type of transformation varies depending on the radioisotope, but some transformations result in a different number of protons, so that the original atom becomes a different element.

Scientists can detect released radiation from *radioisotopes. For instance, if you feed an earthworm food containing a radioisotope, you can follow the worm’s path through the soil by using a detector called a Geiger counter. Most atoms in living organisms are organized into stable associations called molecules. If a radioisotope is incorporated into a molecule, it acts as a tag or label, allowing a researcher or physician to track the molecule in an experiment or in the body (Figure 2.3).

Question

The technique could be used to monitor therapy. As therapy progresses and the patient improves, one would expect that the brain region involved would show more activity.

Although radioisotopes are useful in research and in medicine, even a low dose of the radiation they emit has the potential to damage molecules and cells. However, these damaging effects are sometimes used to our advantage; for example, the radiation from ⁶⁰Co (cobalt-60) is used in medicine to kill cancer cells.

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