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Posted by on Mar 25, 2011 in Physical Chemistry | 0 comments

Nuclear Radiation (1): Radioactive Processes

I thought I should say something about the nuclear reactor disaster in Japan as I know there are a lot of questions about reactors, how they work and how radiation is dangerous.

In this part I will talk a bit about the types of radiation.  In part 2, I will explain how a reactor works, and finally in part 3, discuss some of the effects of radiation on humans.
Fukushima I by Digital Globe 2
Fukushima I by Digital Globe 2 crop

The FUKUSHIMA reactor  sustained considerable damage.

Nuclear Reactions and Nuclear Energy

In the past, the job of the early alchemist was to change elements to gold.  No one could do it, so some used magic,

Alchemists tried to change elements to gold.

This is called transmutation.  Alchemists failed because it is not possible to transmutate elements by  chemical reactions.  You need nuclear reactions and the energy is much greater than chemical energy because of E=mc2.

In radiation processes, an atom’s nucleus undergoes a change in the number of protons and/or neutrons it has.  I will talk about alpha decay, beta decay, positron decay and the emission of gamma rays in a moment, but first, let us express what is going on in radiation by the introducing a plot of neutrons vs protons (see figure below). It can be used to show the changes to the atom that radiation causes. Let us look at a nuclear reaction.

Nuclei are described by a notation shown here. The atomic number is actually redundant because the atomic symbol is given. The atomic weight can vary for different isotopes.

Nuclear notation for Uranium

If you follow through these examples, you can see how nuclear reactions are written.

Nuclear Reaction: Uranium is transmutated to Thorium

As an alpha particle is emitted, its atomic number (2) and mass (4) must be subtracted from Uranium.  Uranium is transmutated to Thorium. An alpha particle is a helium nucleus which has two protons and two neutrons, This is the atomic number is 2 and its the atomic weight is 4

So Uranium 238 (upper spot) decays by alpha decay to Thorium (lower spot).  We can indicate that by a point on the graph below.

Plot of Neutrons vs. Protons showing alpha decay

An alpha particle is a helium nucleus. So it has 2 protons and 2 neutrons with an overall charge of 2+. When an alpha particle is ejected, the mass of the nucleus falls by four atomic units and the charge falls by 2+.

A beta particle is a name given to an electron that is ejected from a nucleus in nuclear decay. When the beta particle is ejected, the number of protons increases by one and the number of neutrons decreases by one.

Plot of beta decay

Image of beta decay

A positron can also be emitted. This has the opposite effect to beta emission. Raising the number of neutrons by one and lowering the number of protons by one.

Electron capture has the same effect as positron emission.

Nuclei have a quantized energy level structure, but the energies involved are many orders of magnitude greater than the energies of valence electrons.  When a nucleus undergoes such a transition, the energy has very short wavelength and lies in the gamma radiation range.

Gamma emission

In the next part I will talk about how nuclear energy is used and discuss the parts of a reactor.

The video above uses clips from the best selling  Introductory Chemistry Tutorial and Physics Tutorials from MCH Multimedia: Learn in depth by working through the modules which are presented with clear verbal instructions and descriptions.  There are many places where you can plot results and do simulations. I am sure it will help you learn Chemistry and Physics faster and better!

You can also find similar topics and their explanations, along with  interactive multimedia animations, in the latest edition of Physical Chemistry e-book by Laidler, Meiser, Sanctuary

~ Bryan

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