Science Time   
                                                                                                                                                      Last updated:  02/09/2010


       

            

     
Sciencetime© 2009
reproduction by permission of author

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


                                                                       Nuclear Decay
 alpha decay
beta decay
positron decay
electron capture

You need to know how to write and understand nuclear symbols

The lower number is the atomic number and the upper number is the mass number.

I.  Alpha Decay

In 1899, Ernest Rutherford wrote the following words:

"These experiments show that the uranium radiation is complex and that there are present at least two distinct types of radiation - one that is very readily absorbed, which will be termed for convenience the alpha-radiation, and the other of more penetrative character which will be termed the beta-radiation."

The image to the right is of a twenty-eight year old Ernest Rutherford while at McGill University in 1899.

Alpha decay can most simply be described like this:

1) The nucleus of an atom splits into two parts.
2) One of these parts (the alpha particle) goes zooming off into space.
3) The nucleus left behind has its atomic number reduced by 2 and its mass number reduced by 4 (that is, by 2 protons and 2 neutrons).
A typical alpha decay equation:

Notice several things about it:
1) The atom on the left side is the one that splits into two pieces.
2) One of the two atoms on the right is ALWAYS an alpha particle.
3) The other atom on the right ALWAYS goes down by two in the atomic number and four in the mass number.

 

Another example:

Check it and compare the three points to the example. Keep in mind that this equation shows the left-hand side splitting into the two pieces shown on the right-hand side.


Practice problems: 
Write the alpha decay equations for these five nuclides. 
Then click the link to see the answers.

Exercise #1:  

II. Beta Decay

Beta decay is somewhat more complex than alpha decay is. A simplified view of what beta decay actually is:

1) A neutron inside the nucleus of an atom breaks down, changing into a proton.
2) It emits an electron and an anti-neutrino (more on this later) which go zooming off into space.
3) The atomic number goes UP by one and mass number remains unchanged.

A beta decay equation:

Some points to be made about the equation:

1) The nuclide that decays is the one on the left-hand side of the equation.
2) The order of the nuclides on the right-hand side can be in any order.
3) The way it is written above is the usual way.
4) The mass number and atomic number of the antineutrino are zero and the bar above the symbol indicates it is an anti-particle.
5) The neutrino symbol is the Greek letter "nu."

Another example of a beta decay equation:

Notice that all the atomic numbers on both sides ADD UP TO THE SAME VALUE and the same for the mass numbers.

Practice problems:Write out the full beta decay equation. 
Then click the link to see the answers.

Exercise #2:


 

III. Positron Decay

Positron decay is like a mirror image of beta decay. A simplified view of what positron decay actually is:

1) Something inside the nucleus of an atom breaks down, which causes a proton to become a neutron.
2) It emits a positron and a neutrino which go zooming off into space.
3) The atomic number goes DOWN by one and mass number remains unchanged.

An example of a positron decay equation:

Some points to be made about the equation:

1) The nuclide that decays is the one on the left-hand side of the equation.
2) The order of the nuclides on the right-hand side can be in any order.
3) The way it is written above is the usual way.
4) The mass number and atomic number of the neutrino are zero.
5) The neutrino symbol is the Greek letter "nu."

Another example of a positron decay equation:

Notice that all the atomic numbers on both sides ADD UP TO THE SAME VALUE and the same for the mass numbers.

Practice problems: Write out the full positron decay equation. 
Then click the link to see the answers.

Exercise #3:



                                                         IV.  Electron Capture

Electron capture is not like any other decay - alpha, beta, or position. All other decays shoot something out of the nucleus. In electron capture, something ENTERS the nucleus. A simplified view of what electron capture is:

1) An electron from the closest energy level falls into the nucleus, which causes a proton to become a neutron.
2) A neutrino is emitted from the nucleus.
3) Another electron falls into the empty energy level and so on causing a cascade of electrons falling. One free electron, moving about in space, falls into the outermost empty level. (Incidently, this cascade of electrons falling creates a characteristic cascade of lines, mostly (I think) in the X-ray portion of the spectrum. This is the fingerprint of electron capture.)
4) The atomic number goes DOWN by one and mass number remains unchanged.

An electron capture equation:

Some points to be made about the equation:

1) The nuclide that decays is the one on the left-hand side of the equation.
2) The electron must also be written on the left-hand side.
30 A neutrino is involved , It is ejected from the nucleus where the electron reacts, so it is written on the right-hand side. 3) The way it is written above is the usual way.

Another example of a electron capture equation:

Notice that all the atomic numbers on both sides ADD UP TO THE SAME VALUE and the same for the mass numbers.

Practice problems:  Write out the full electron capture equation. 
Then click the link to see the answers.

Exersice #4: