Question

Why is fusion exothermic?

Answer 1

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 = mc^2`).

A fusion reaction is exothermic.