A nuclear reaction<span> in which two or more atomic nuclei come very close and then collide at a very high speed and join to form a new type of atomic nucleus.
      Hope this helps :)
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<u>Answer:</u> The original element is 
<u>Explanation:</u>
Alpha decay is defined as the process in which alpha particle is emitted. In this process, a heavier nuclei decays into a lighter nuclei. The alpha particle released carries a charge of +2 units.
The released alpha particle is also known as helium nucleus.

For the given alpha decay process of an isotope:

<u>To calculate A:</u>
Total mass on reactant side = total mass on product side
A = 208 + 4
A = 212
<u>To calculate Z:</u>
Total atomic number on reactant side = total atomic number on product side
Z = 82 + 2
Z = 84
The isotopic symbol of unknown element is 
Hence, the original element is 
 
        
             
        
        
        
Answer:
Look at the periodic table of elements 
Explanation:
Group 1 is the most reactive and to the right
Group 18 is least reactive and all the way to the left. The rest are in between. The groups go vertical. 
You'd be able to solve with this information youre welcome