# Why reduced atoms are more energetic

For example, in glycolysis, NAD+ is reduced to NADH and then it's more energetic. Why adding electrons to an atom gives it more energy?

Particularly, since $$E=(mc^2)^2+(pc)^2$$ any object or particle of some mass or momentum must have energy. By this rudimentary logic, any atom containing more mass (and electrons have 9.91e-31 kg of it) should have more energy.