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Nuclear Reactions
Nuclear Reactions
The atomic number of an element is the number of protons in a nucleus of one of its atoms.
 
The mass number of a nucleus is the total number of protons plus neutrons it possesses.
 
Nuclei of a given element having different numbers of neutrons are said to be different isotopes of that element.
When describing atomic nuclei, we use the chemical symbol for the element concerned plus the atomic number of the element and the mass number of the isotope being considered.
 
For example
 
represents a nucleus of the element uranium having 92 protons and 143 (235-92) neutrons.
The next symbol represents another isotope of the same element  
 
 
which has three more neutrons. 
 
When writing nuclear equations we will use two conservation principles:
We will assume that during any nuclear reaction, the total atomic number and the total mass number of the nuclei remain constant.
 
Example 1
A neptunium nucleus decays into a plutonium nucleus by emitting radiation.
 
The neptunium nucleus is represented by
The plutonium nucleus is represented by
Therefore, the reaction can be written as
 
The atomic number has increased by 1 and the mass number has not changed so the radiation must be beta radiation which means that the full equation is
but you might also see
because a beta particle is the same as an electron.
 
Example 2
A uranium-235 nucleus decays into a thorium nucleus by the emission of an alpha particle
also written as  
since an alpha particle is the same as a helium nucleus. 
From this we can say that the atomic and mass numbers for the thorium nucleus must be as in the next equation
 
Example 3
A nucleus of uranium-235 can split into (various pairs of) other lower mass nuclei. 
This process is called nuclear fission.  
The next (partial) equation represents one possible nuclear fission reaction in which nuclei of barium and krypton are produced.
 
 
We will again use the conservation principles to fill in the blanks. 
The first missing symbol must be uranium because the atomic number is still 92.
Therefore 
 
 
The brackets show that this isotope is unstable and decays soon after the arrival of the neutron. 
You will notice that, on the right of the second arrow, we still have atomic number 92 but are missing three in the mass number total.
Therefore, the final equation for this reaction must be  
 
 
Notice that this reaction was initiated by the arrival of a neutron and that the reaction also emits three neutrons.
These three neutrons might go on to cause similar reactions in other uranium nuclei.  
If this occurs, we say we have started a chain reaction, as represented by the diagram below. 
 
Under the right conditions (or the wrong conditions, depending on your point of view), this can lead to a very rapid release of energy known as... a nuclear explosion!
 
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