Radioactive elements varying atomic weights

Lead, atomic number 82, is a member of Group 14 (IVA) of the Periodic Table. Ordinary lead is bluish grey and is a mixture of isotopes of mass number 204 (15%), 206 (23.6%), 207 (22.6%), and 208 (52.3%). The average atomic weight of lead from different origins may vary as much as 0.04 units. The stable isotopes are products of decay of three naturally radioactive elements (see Radioactivity, natural) comes from the uranium series (see Uraniumand... 

With atomic number 92 and atomic weight 238.03, uranium is the heaviest naturally occurring element. There are eleven known isotopes, of which three — with atomic weights 234, 235 and 238 — occur in nature. They are all radioactive, with half-lives (in years) of 2.35 X 10 , 7 X 10 and 4.5 X 10 , respectively. The relative abundance of the isotopes varies depending on the age and geological history of the uranium occurrence a typical distribution is U 99.28% U 0.71% U 0.005%. A quite exceptional deviation from this distribution has been found at Oklo in Gabon as a result of spontaneous fission chain reactions in the remote past (Anon., 1975). At Oklo U concentrations as low as 0.29% have been found. 

The atomic weight of an element comes from the nucleus. Protons and neutrons in the nucleus each have a weight of 1 AMU. Atoms of elements have varying numbers of protons and neutrons. The total number of neutrons and protons in each atom equals the atomic weight. The atomic number of an element equals the number of protons in that element. The number of neutrons is determined by subtracting the number of protons from the atomic weight what remains is the number of neutrons. In chemistry, we are concerned more with the electrons orbiting the nucleus than the nucleus itself. We are particularly interested in the electrons in the outer shell of the atom. Chemical activity takes place between the outer-shell electrons of elements this chemical activity forms compounds. In radioactivity, the concern is with the nucleus, where radiation is emitted. Radioactivity will be discussed further in Chapter 8. 

Uranium is a radioactive element. It is found in several different elemental forms whose atomic weights vary. When one element has atoms with different atomic weights, each different weight is known as a separate isotope of the element. To understand the fission process, it is important to understand the basic structure of the atom, the different isotopes of uranium and plutonium, and why some of these isotopes are radioactively unstable. 

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