Another use of radioactive carbon-14 is to date prehistoric bones and teeth, the longest-surviving parts of the body. Carbon-14 (¹⁴C; six protons and eight neutrons) is continually formed in the upper atmosphere by cosmic rays acting on nitrogen (¹⁴N; seven protons and seven neutrons). The energy of the cosmic rays causes a proton and electron in atoms of ¹⁴N to fuse into a neutron. The newly formed ¹⁴C is rapidly oxidized to ¹⁴CO₂, which enters the earth’s plant and animal life through photosynthesis and the food chain. The ratio of ¹⁴C to nonradioactive carbon is approximately constant over time. When plants or animals die, carbon uptake ceases, ¹⁴C is not replenished, and it slowly decays with a half-life of close to 5,700 years. The radioactivity of a sample whose age is not known can be used to indicate the amount of ¹⁴C remaining, provided it is not more than 40,000 years of age. After that time, so much ¹⁴C has decayed that what is left is not measurable. From the ratio of radioactive carbon to total carbon content, it is possible to determine how much was lost relative to the current ratio (unchanged over billions of years) and therefore how long ago the bone or tooth was part of a living organism.

Not all isotopes are radioactive, and even some stable isotopes can be useful. A stable isotope of carbon, carbon-13 (¹³C; 6 protons and 7 neutrons) has accounts for about 1% of carbon atoms on earth. Because it is chemically more stable than ¹²C, living organisms preferentially utilize ¹²C for chemical reactions (metabolism). Therefore, rocks containing a greater than usual ¹²C/¹³C ratio are potentially ‘chemical fingerprints’ of life. Minute residues in some of the oldest rocks on Earth, from Akilia Island near Greenland, have a chemical fingerprint that may have come from living organisms. Analysis of selected tiny samples using an ion microprobe revealed ratios of carbon-13 to carbon-12 that were 2–5% less than expected. Some prebiotic process may have enriched the rock with carbon-12 atoms, and it lay apparently undisturbed (based on the surrounding crystal structure) for over 3,700 million years (3.7 Giga-years, Gyr).

The age of ancient rock samples is independently determined from the rate of decay of another isotope, ⁸⁷Rubidium (⁸⁷Rb) to ⁸⁷Strontium (⁸⁷Sr). The half-life of this decay is about 1.0 Gyr. Amounts of ⁸⁷Sr and ⁸⁷Rb are measured, and a ratio is derived and compared with the ratio of ⁸⁷Sr to stable strontium (⁸⁶Sr) in the rock sample. The age of the rock sample is then derived mathematically from these ratios, given the half-life of ⁸⁷Rb.

The common element of oxygen has eight protons and eight neutrons. A stable isotope with two additional neutrons is also relatively common. Because water-containing ¹⁸O evaporates more slowly and condenses more rapidly, the ratio of ¹⁸O/¹⁶O in Antarctic ice cores indicates major climate shifts since early in geologic time. Increased ¹⁸O indicates a sudden warming, and increased ¹⁶O indicates a sudden cooling.

The electrons of the elements are arranged in shells surrounding the nucleus. The first shell consists of two electrons, the second and third each of eight electrons, and the fourth and fifth each of 18 electrons. Elements take part in a chemical reaction by gaining or sharing electrons to complete their outer shell except for the noble gases (helium, neon, argon, krypton and xenon, far right-hand column of the periodic table). These elements already have a complete outermost electron shell. They therefore have no chemical reactivity and are incompatible with life.

Figure 1.1 shows the elements important for all of life. The common elements are depicted in dark green boxes: carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur. Elements depicted in light orange/yellow or pink/white boxes make up the major cations and anions of living organisms: sodium, potassium, magnesium, calcium, manganese, iron, cobalt, nickel, copper, zinc, chlorine, and iodine. Trace elements (light green, pink, or red boxes) occur as occasional enzyme cofactors: boron, fluorine, silicon, arsenic, selenium and bromine, aluminum, gallium, chromium, vanadium, molybdenum, tungsten, and cadmium.

Fluorine is present on earth only as fluoride, a negatively charged anion that is especially important in dentistry because of its ability to mediate protection from dental caries (Chapter 16, section 16.2.1.). Although metabolites of chlorine and iodine are ubiquitous, few biological products contain fluoride because of its tight hydration shell. The few enzymes that utilize fluoride as a cofactor must overcome an exceptionally high desolvation energy barrier.

The fluoride content of a solution is measured with an electrode made from a mixture of lanthanum and europium fluorides. Lanthanum follows barium in the periodic table and has an atomic number of 57. Lanthanum is also the first of a series of 15 heavy metals before hafnium called lanthanides. Europium is a lanthanide and its atomic number is 63. The mode of action of the fluoride electrode is described in Chapter 16, sect. 16.1.1.