Step-by-step 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