Elements disintegration

A radioactive element disintegrates at a rate that is independent of any external conditions. The number of disintegrations occurring per unit time is proportional to the number of atoms originally present. This can be formulated as a first-order differential equation in time ... 

If radioactive elements disintegrate, how is it possible that they still remain in the earth s crust after 4.5 billion years, which is estimated to be the age of the earth Yet, some are not left - the exciting story about the search for element 43, technetium, tells us that (Chapter 28 Technetium). It is the same story for the rare earth element 61, promethium (Chapter 17 Rare earths). These elements may be found after reactions in a nuclear reactor, but they are short-lived. The most long-lived technetium isotope has a half-life of 4 million years the most long-lived promethium isotope only 17.7 years. If they had been present at the creation of the earth, they would have disappeared long ago. 

Atoms with the same number of protons but a different number of neutrons are called isotopes. To identify an isotope we use the symbol E, where E is the element s atomic symbol, Z is the element s atomic number (which is the number of protons), and A is the element s atomic mass number (which is the sum of the number of protons and neutrons). Although isotopes of a given element have the same chemical properties, their nuclear properties are different. The most important difference between isotopes is their stability. The nuclear configuration of a stable isotope remains constant with time. Unstable isotopes, however, spontaneously disintegrate, emitting radioactive particles as they transform into a more stable form. 

Atoms with the same value of Zbut different values of A are isotopes (Table 11.1). Many isotopes are stable but others are naturally or artificially radioactive, i.e. their atomic nuclei disintegrate, emitting particles or radiation. This changes the nuclear structure of the atom and often results in the production of a different element. 

Decay The spontaneous disintegration of an unstable atomic nucleus to form another more stable element or isotope of a lower atomic mass. 

Radioactive materials Elements that have unstable nuclei that spontaneously disintegrate, releasing radiation in the form of subatomic particles and energy. 

E. Rutherford (Manchester) investigations into the disintegration of the elements and the chemistry of radioactive substances. 

N are more or less disintegrated, yielding very diffuse diffractions, apparently due in part to very small crystalline elements of mercerized cellulose... 

Today, modern instrumentation provides much more direct evidence of atoms (Fig. B.3). There is no longer any doubt that atoms exist and that they are the units that make up the elements. In fact, chemists use the existence of atoms as the definition of an element an element is a substance composed of only one kind of atom. By 2006, 111 elements had been discovered or created but in some cases in only very small amounts. For instance, when element 110 was made, only two atoms were produced, and even they lasted for only a tiny fraction of a second before disintegrating. 

When Z gets big enough, no number of neutrons is enough to stabilize the nucleus. Notice in Figure 2-20 that there are no stable nuclei above bismuth, Z — 83. Some elements with higher Z are found on Earth, notably radium (Z = 88), thorium (Z = 90), and uranium (Z = 92), but all such elements are unstable and eventually disintegrate into nuclei with Z < 83. Consequently, the set of stable nuclei, those that make up the world of normal chemistry and provide the material for all terrestrial chemical reactions, is a small subset of all possible nuclei. 

Secular Equilibrium—If a parent element has a very much longer half-life than the daughters (so there is not appreciable change in its amount in the time interval required for later products to attain equilibrium) then, after equilibrium is reached, equal numbers of atoms of all members of the series disintegrate in unit time. This condition is never exactly attained, but is essentially established in such a case as 226Ra and its transformation series to stable 206Pb. The half-life of 226Ra is about... 

Radioisotope—An unstable or radioactive isotope of an element that decays or disintegrates spontaneously, emitting radiation. Approximately 5,000 natural and artificial radioisotopes have been identified. 

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