Question

(a) What do you mean by $Q$ value of a nuclear reaction?
(b) Write down the expression for $Q$ value in the class of $\alpha$ decay.

Answer 1

a) The Q-value of the reaction is defined as the difference between the sum of the masses of the initial reactants and the sum of the masses of the final products, in energy units (usually in MeV). It is the measure of amount of energy released by that reaction.
b) The Q-value of the decay, $Q_{\alpha }$ is the difference of the mass of the parent ($m_P$) and the combined mass of the daughter (($m_P$) and the $\alpha$-particle ($m_{\alpha }$), multiplied by $c^2$:

$Q_{\alpha }=(m_P-m_D-m_{\alpha })c^2$.