Isotopes of lead

Lead (13 ppm) is by far the most abundant of the heavy elements, being approached amongst these only by thallium (8.1 ppm) and uranium (2.3 ppm). This abundance is related to the fact that 3 of the 4 naturally occurring isotopes of lead (206, 207 and 208) arise primarily as the stable end products of the natural radioactive series. Only (1.4%)... 

The products in this long sequence of reactions accumulate in the stable isotope of lead, 206Pb. The amount of 206Pb present depends upon how long the deposit of uranium has decayed since the crystal U30 was formed. 

A useful analogy for understanding secular equilibrium is visualizing a decay chain as a series of pools of water (Fig. 2). These pools eventually lead to a continuously filling pool representing a stable isotope of lead (either ° Pb, ° Pb or ° Pb). Over the timescale... 

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

The four isotopes, as those of any element, have the same chemical properties. The four are not, however, uniformly distributed in the earth s crust the occurrence of three of them, in minerals and rocks, is associated with the radioactive decay of isotopes of thorium and uranium. In most minerals and rocks the relative amounts (or the isotopic ratios) of the isotopes of lead (often expressed relative to the amount of stable lead-204) are generally within well-known ranges, which are independent of the composition of the mineral or rock they are, however, directly related to the amounts of radioactive thorium and uranium isotope impurities in them. 

Minerals and rocks of similar composition but of different geographic or geologic origin generally include different relative amounts of thorium and uranium impurities after generally long periods of time they also include, therefore, different relative amounts of the isotopes of lead. The... 

The four, naturally occurring isotopes of lead are listed in Table 12 along with their percent natural abundance173. 

ICP-MS spectrum showing the naturally occurring isotopes of lead. 

According to the model, this isotopic composition was fixed tm years ago (on the separation of the lead minerals from the uranium- and thorium-bearing environment), is unchanged to the present day and is therefore measurable. There are two similar equations for the other two radiogenic stable isotopes of lead. To simplify the manipulation, we can use the following notation ... 

Figure 9.5 ICP-MS survey data from masses 203 to 210. The vertical columns show the expected positions and relative abundances of the four natural isotopes of lead (204Pb, 1.4% 206Pb, 24.1% 207Pb, 22.1% 208Pb, 52.4%). The agreement between the survey data (dotted line) and the actual abundance of lead confirms that lead is present, and that there are no significant interfering elements.

From Figs. 1.1 and 1.2 it follows that the main end products of the uranium and thorium series are isotopes of lead (at the time referred to as Pb206.5 and ThO2208.4). The end products are thus isotopes of lead differing by two mass units. This observation became the motivation for the measurement of atomic weights of lead samples separated from thorium and uranium minerals. In his Nobel Lecture, Soddy describes this work as follows ... 

Radium is extremely radioactive. It glows in the dark with a faint bluish light. Radiums radioisotopes undergo a series of four decay processes each decay process ends with a stable isotope of lead. Radium-223 decays to Pb-207 radium-224 and radium-228decay to Pb-208 radium-226 decays to Pb-206 and radium-225 decays to Pb-209. During the decay processes three types of radiation—alpha (a), beta ((5), and gamma (y)—are emitted. 

ISOTOPES There are 47 isotopes of lead, four of which are stable. One of these four is Pb-204, which makes up 1.4% of the natural abundance of lead found on Earth. In reality this isotope is not stable but has a half-life that is so long (1.4x10+ years), with some of the ancient deposits still existing, that it is considered stable. The other three stable isotopes of lead and their proportion to the total natural abundance are as follows Pb-206 = 24.1%, Pb-207 = 22.1%, and Pb-208 = 52.4%. All the other isotopes are radioactive. 

Uranium is the fourth metal in the actinide series. It looks much like other actinide metallic elements with a silvery luster. It is comparatively heavy, yet malleable and ductile. It reacts with air to form an oxide of uranium. It is one of the few naturally radioactive elements that is fissionable, meaning that as it absorbs more neutrons, it splits into a series of other lighter elements (lower atomic weights) through a process of alpha decay and beta emission that is known as the uranium decay series, as follows U-238—> Th-234—>Pa-234—>U-234—> Th-230 Ra-226 Rn-222 Po-218 Pb-2l4 At-218 Bi-2l4 Rn-218 Po-2l4 Ti-210—>Pb-210—>Bi-210 Ti-206—>Pb-206 (stable isotope of lead,... 

Most of the chemical and physical properties of imniloctium (hassium) are unknown. What is known is that its most stable isotope (hassium-108) has the atomic weight (mass) of about 277. Hs-277 has a half-life of about 12 minutes, after which it decays into the isotope seaborgium-273 through either alpha decay or spontaneous fission. Hassium is the last element located at the bottom of group 8, and like element 107, it is produced by a cold fusion process that in hassium s case is accomplished by slamming iron (Fe-58) into particles of the isotope of lead (Pb-209), along with several neutrons, as follows ... 

---