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Chain reaction
 
This model illustrates the different stages in the chain reaction between hydrogen and chlorine as these two elements are exposed to light. The reaction begins with a chlorine molecule absorbing a light particle that has a sufficiently high energy to break up the chlorine molecule into two separate atoms (initiation stage). The separated chlorine atom has an unpaired electron; the number of its electrons is only one electron less than the total number required for the noble gas configuration. Consequently, the chlorine atom's level of chemical activity is very high (it is a radical ion). The chlorine atom is capable of breaking down a hydrogen molecule, and attaching to one of the hydrogen atoms, thus forming a molecule of HCL.

The second hydrogen atom is left free. It has one unpaired electron. If such an atom collides with a molecule of chlorine, it detaches one of the chlorine atoms from this molecule in the same fashion that the chlorine atom has unpaired the original hydrogen molecule. The second chlorine atom is left free; and the abovementioned process repeats. This process is called a chain growth stage. But if the two free atoms interact to form a molecule (in our model, an atom of hydrogen and an atom of chlorine play this role), no radical ions are left over after this process takes place. The chain reaction does not continue. That is why this stage in the chain reaction is called the chain break stage.

 
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