Discuss why it is impossible for a particle to decay into a ...

[gbarreiro]

Discuss why it is impossible for a particle to decay into a single particle of less mass.
 
  What will be an ideal response?

Question 2

Why do the high-proton-number elements tend to get more unstable?
 
  What will be an ideal response?

[fromAlphatoOmega22]

Answer to Question 1

Observe the decay from the rest frame of the original particle, in which momentum is obviously zero. If a single particle were created that has less mass, then some of the original mass became kinetic energy and the final particle would be moving and have momentum. But momentum must be conserved and the final can not have momentum.

Answer to Question 2

The repulsive electrostatic force between protons increases progressively as atomic number increases, but the nuclear attractive force is short range and the average distance between particles increases as the nucleus size increases.

[gbarreiro]

Thank you!

[fromAlphatoOmega22]

Always glad to help...