# Why is fusion exothermic?

May 5, 2014

Fusion is exothermic because the mass of the products is lower than the mass of the two reacting nuclei.

Here are some typical masses:

• neutron = 1.009 u
• deuterium nucleus = 2.014 u.
• tritium nucleus = 3.016 u
• helium nucleus = 4.003 u

In a fusion reaction, two light nuclei fuse together and form a heavier nucleus.

The mass of the products is less than the mass of the two reacting nuclei.

For example, deuterium and tritium nuclei fuse to form a helium nucleus (α particle) and a neutron.

The difference in masses is

(α + n) – (D + T) = (4.003 u + 1.009 u) – (2.014 u + 3.016 u) = 5.012 u – 5.030 u = -0.018 u.

The product nuclei have less mass than the reactants.

This difference in mass is given off in the form of energy ($E = m {c}^{2}$).

A fusion reaction is exothermic.