Answer
Lithium, $^7_3Li$
Work Step by Step
Assume $^A_ZX$ to be the unknown particle with mass number (A) and atomic number (Z).
Important things to take note in the reactions are
1. Particle X absorbed a neutron,
2. Particle X emits an electron
3. New nuclide splits spontaneously into two alpha
particles.
From this information we can construct an equation
$^A_ZX + ^1_0N \rightarrow ^{\space \space \space 0 }_{-1} e + 2 \space ^4_2 He $
Add up all the charges $Z + 0 = -1 + 4 = 3$
Add up all the mass number $A +1 = 0 + 8 = 7$
An element with A =7 and Z = 3 is Lithium, $^7_3Li$
