In nuclear and particle physics, the **energetics of nuclear reactions** is determined by the **Q-value** of that reaction. 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).

Consider a typical reaction in which the projectile a and target A give place to two products, B and b. This can also be expressed in the notation we have used, **a + A → B + b**, or even in a more compact notation, **A(a,b)B**.

See also: E=mc^{2}

The **Q-value** of this reaction is given by:

**Q = [m _{a} + m_{A} – (m_{b} + m_{B})]c^{2}**

which is the same as the **excess kinetic energy** of the final products:

**Q = T _{final} – T_{initial}**

** = T _{b} + T_{B} – (T_{a} + T_{A})**

For reactions in which there is an increase in the kinetic energy of the products, **Q is positive**. The positive Q reactions are said to be **exothermic** (or **exergic**). There is a net release of energy since the kinetic energy of the final state is greater than the kinetic energy of the initial state.

For reactions in which there is a decrease in the kinetic energy of the products, **Q is negative**. The negative Q reactions are **endothermic** (or **endoergic**), and they require net energy input.