Promethium An Introduction to Nuclear Chemistry

We consider that one of the essential parts of physical chemistry is some awareness of nuclear chemistry. While the periodic chart poses as a list of stable elements, there are hints of irregularity by the absence of elements no. 43 (technetium, Tc) and no. 61 (promethium. Pm). Modem students are also aware of unstable elements beyond no. 92 (uranium, U). Since nuclear reactions seem to follow a sequence of first-order reactions, we take some time to mention a few of the mechanisms that are occurring in the first-order processes. 

The main point here is that the case of Pm shows that while nuclear decay is usually thought of as decreasing to a lower atomic number, some reactions (beta decay, a form of electron emission) actoally increase the atomic number of the elemental species. Note that in either case, we can use 

Let us consider the decay of Pm by two mechanisms (1) electron capture where a Is electron is pulled into the nucleus to make a neutron from one of the protons and (2) beta emission where an electron is ejected and a neutron decays into a proton. Neutrons are not stable and when they are out of a nucleus they have a half-life of 10.3 min but what determines whether a proton becomes a neutron or a neutron becomes a proton depends on the other protons and neutrons in a given nucleus. 

With that very brief exposure to the complexity of nuclear chemistry and perhaps a better appreciation of the significance of the research of the Curies, let us consider the simplest case  

we see that the first-order kinetic equation is much simpler than the consideration of the decay mechanism. From a geological point of view, this shows that any primal amount of Pm would have rapidly decayed in the lifetime of the Earth of over 4 billion years. In fact the age of the Earth has been estimated by the presence or absence of radioactive isotopes in the cmst of the Earth related to their isotopes, although the original amounts have to be estimated. Nuclear decay is a prime example of first-order kinetics and as with the case of any kinetic problem, the decay is totally dependent on 

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