Question

²³⁵U decays by emitting an ? particle and forming ²³¹Th. What is the energy of the emitted ?, in MeV (3 sig figs)?  

Answer 1

The decay equation can be written as:

²³⁵U₉₄ --->   ²³¹Th₉₂ + ⁴He₂

Atomic mass of U-235 is 235.043929918 u
Atomic mass of Th-231 is 231.036304343 u
Atomic mass of He-4 is 4.002603254 u

The total mass on the left side of the equation is 235.043929918 u.
The total mass on the right side of the equation is 231.036304343 u + 4.002603254 u = 235.038907597 u

The left side of the equation has more mass, by 0.005022321u.

Since 1u = 931.5 Mev/c²

Q value = 0.005022321 x 931.5 = 4.6782920115 Mev

K. E of alpha particle = [(Mass Number after decay) /(Mass number before decay)] x Q

= (231 / 235) x Q = 4.599 Mev