These differences are purely a function of the electronegativities of F, O, and Cl, that is, the strength with which each atom pulls electrons toward itself.

After combustion, the electrons that were shared equally between carbon and hydrogen in methane are drawn toward the Cl atoms when Cl₂ is the combustion medium, drawn more strongly toward O atoms, and most strongly of all toward F when F₂ is the combustion medium.

The tighter the electrons are held after combustion, the more stable the product molecules are, and the more energy is given off as heat.