Alpha decay please help (nuclear chemisty)

Explanation:

Given reaction:

           ¹⁰⁸₅₃I   →  ⁴₂He   +  ?

 An alpha particle decay requires that the mass number decreases by 4 and atomic number decreases by 2.

This reaction is called  nuclear fission;

  The unknown compound, say Y, will have mass number and atomic number shown below;

               ¹⁰⁸₅₃I   →  ⁴₂He   +  ᵃₙY

Mass number;                  108  =  4 + a

                                               a = 104

Atomic number ;

                                            53  = 2 + n

                                               n = 51

   Y will be      ¹⁰⁴₅₁Y

The atomic number of the new element is 51; so, it is antimony.

               ¹⁰⁴₅₁Sb

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