South Dade Senior High School Mr. Sackman

 What is the first thing that comes to your mind when you think of the word nuclear?  I have done a poll and 98% of all student’s first response is “explosions, or bombs”.  Yes enormous explosions are created with nuclear chemistry, but what does the term really mean?  Look at the word nuclear, change the last two letters to “us” instead of “er” and there is the first part of your answer.

 The term nuclear does refer to the nucleus of an atom and the word nuclear chemistry is the study of changes in atomic nuclei.  Remember atomic nuclei, or a nucleus, is made of protons and neutrons, these subatomic particles can be termed as nucleons.  In nuclear chemistry an atom can be called a nuclide is identified as by the number of protons and neutrons in its nucleus? Do you remember which number on the periodic tablet this is?

 You would expect to take the masses of all subatomic particles and add them together to get the complete mass of an atom, is this really the case? Why or why not?  As you will see this is not the case, but who is this possible knowing the law of conservation on mass?

 Take the Helium (He) atom as an example remembering that in fact an electron has a tiny bit of mass but compared to a proton or neutron, which are both more massive, it can be considered to have none.  Here are the exact values of the mass of subatomic particles in amu

ParticleMass (amu) Electron amu Neutron amu Proton amu

 Helium has two protons, two electrons, and two neutrons so lets add the masses of each together 2 protons: (2 X amu) 2 neutrons: (2 X amu) 2 electrons: (2 X amu) total combined mass: amu  The literature, or measured current value, is , this is amu different then he calculated value. This difference in mass is called mass defect

 So what causes this loss in mass, think of a very famous science equation that everyone has heard of or seen before in their life.  E=mc2, where E = energy, m- mass, and c= the speed of light.  Einstein believed that according to this equation mass can be converted to energy, which would explain for mass loss  If we take that mass defect value and plug it into the above equation, what does that value for energy represent?

 This energy is called the nuclear binding force  What does this binding force represent, remember what particle is in the nucleus and what charges they are?  This energy can be considered to be the amount of energy needed to form a nucleus from nucleons.  When nucleons come together they convert some of their mass to energy which enables the nucleus to remain intact

 This energy can also be considered to be a measure of the stability of the nucleus.  The higher the binding energy the more tightly the nucleus is held together  To find the binding energy per nucleon simply divide the value for energy by the number of nucleons.

 When the number of protons are plotted against the number of neutrons for stable nuclei, a belt like graph is obtained with two lines  The top line refers to a n/p ratio of 1.5:1, the bottom refers to a n/p ratio of 1.0:1  Any atom with a n/p ratio between these two lines when the protons and neutrons are plotted is considered to be stable

 In the nucleus there are two forces fighting each other  One is the electrostatic repulsion, is a repulsive force created by like charges being so densely packed next to each other attempting to rip the nucleus apart.  The other force is the nuclear binding force, or strong nuclear force, which is a force between neutrons and protons binding them together.

 If the number of protons in the nucleus begins to increase, as it does as we go higher in the periodic table, the repulsive force increases faster than the nuclear force  More neutrons are and would be required to increase the strong nuclear force so that the nucleus wont become what is called unstable  Every element beyond atomic number 83, Bi (Bismuth), is unstable, or radioactive, because that repulsive force is so great the nuclides can not be stable.

 So what does all the past information on the number of protons and neutrons in a nucleus, and the two competing forces, have to do with a nuclear reactions, or processes?  What happens when the electrostatic repulsive force begins to over power the strong nuclear force, the nucleus becomes unstable, what does this have to do with a nuclear reaction?

 When something is unstable in this universe what does it intend to accomplish?  A nuclear reaction is obviously what?  A reaction that effects the nucleus of an atom  What is this nuclear reaction attempting to accomplish because it is unstable?

 Henri Becquerel was researching a possible connection between light emission and uranium compounds after they were exposed to some form of electromagnetic radiation.  He placed a photographic plate in a light proof container and place a uranium compound on top of the container.  After placing them in the sunlight he noticed the photographic plate was exposed, even though no sunlight hit it, suggesting x-rays hit the plate that were emitted from the compound.  He then tried his experiment again on a cloudy day when there was no sunlight and got the same results, what does this mean?

 This meant that sunlight was not required to produce the rays that struck the photographic plate  Along with Becquerel’s research the, Curies, later termed this process of emission of rays radiation and it was caused by radioactive decay of a unstable nucleus.  The Curies later found that in 1898 only two of the known elements, uranium and thorium were radioactive, they later discovered polonium, and radium to be as well.  Both won a Nobel Prize for their work with radioactive materials. Marie Curie later became the recipient of another Nobel Prize and became the first person to receive a noble prize in both Chemistry and Physics.

 So we know a nucleus will undergo radioactive decay emitting radiation in the process, but how and what types are there?  We will first exam alpha emission, α  Remember Rutherford and the Gold Foil Experiment? What kind of radiation was he shooting at gold foil?  An alpha particle consist of 2 protons, and 2 neutrons bound together and rays of these particles can be emitted when a nucleus undergoes decay.

 If an alpha particle consist of 2 protons, and 2 neutrons what information can you deduce from those facts?  It is equivalent to a Helium atom because Helium has a mass of 4 amu and has 2 protons and can be expressed as either, α, or 4 He 2  It has a positive charge of 2+ due to the 2 protons  This type of emission mostly occurs in heavy nuclei where both the number of protons and neutrons need to be reduced in the n/p ration in order to obtain stability

 One example is the following  Po-210  Pb He-4  Can you complete the following  U-234  ? + He-4  Rn-226  Ra ?  Po-212  He-4 + ?  Es-253  ? + ?

 Nuclides that are above the band of stability are unstable due to the high n/p ratio thus the number of neutrons must be reduced  Beta emissions or Beta Decay will reduce the n/p ration by reducing the number of neutrons.  An example is C-14 decays into N-14  How is it that have the same masses but a different number of protons, what could have occurred for this to happen?

 The reason is in the nucleus a neutrons converts to a proton and an electron.  Since these two are equal in mass the mass number wont change but when changing the number of protons the element will  When this process occurs in the nucleus a electron is emitted from the nucleus and is called a beta particle

 Gamma Radiation is a form of electromagnetic radiation meaning it travels as a wave and has a high frequency, high energy, and low wavelength  These are very high energy  Gamma emission will not occur by itself with an unstable nucleus it follows other decay processes such as alpha, or beta decay  This means when an unstable nucleus decays and emits a high energy wave

 With elements that have low n/p proton ratios and thus low below the band of stability the number of neutrons must be increased there are two ways this occurs  Positron Emission  Electron Capture

 This process is almost the reverse of beta emission  In this process a neutron and a positron are produced from a proton  p  n + β  A positron is a particle with the exact opposite charge as an electron, or beta particle,

 This process changes the number of neutrons by increasing it, and at the same time, it changes the element by decreasing the atomic number  Kr-38  Ar-38  This mass stayed the same why, and why did the element decrease

 Here an unstable nucleus with a low n/p ratio pulls in a electron, hence its name, combines it with a proton to create a neutron  Here the mass stays the same and the atomic number decreases changing the element  Ag e-  Pd-106

 All elements that decay, or change the nucleus, through nuclear process does so at a certain constant rate  The time it take for exactly half of the mass of the unstable element to decay into its stable elements is called the half life  No two radioactive isotopes have the same half life

 Some have lives are extremely short and some are extremely long.

 For example, the half-life of the radioisotope strontium-90 is 29 years  If you had 10.0 g of strontium-90 today, 29 years from now you would have 5.0 g left.  The decay continues until negligible strontium- 90 remains.  Will all the radioactive element ever completely concert into stable products?

The graph shows the percent of a stontium-90 sample remaining over a period of four half-lives. With the passing of each half- life, half of the strontium-90 sample decays.

 Iron-59 is used in medicine to diagnose blood circulation disorders.  The half-life of iron-59 is 44.5 days.  How much of a mg sample will remain after days?

 This can be solved two ways  One you can just simply figure out how many half lives passed by dividing decay time by time of half life and then dividing the intial amount by half and that amount be half until you have divided by two the same number of times as there is half lives or simply remember the following equation

In the equation, n is equal to the number of half-lives that have passed. Note that the initial amount may be in units of mass or number of particles. A more versatile form of the equation can be written if the exponent n is replaced by the equivalent quantity t/T, where t is the elapsed time and T is the duration of the half-life. The time units for both T’s has to be the same