Define exothermic reaction.
Define endothermic reaction.

Exothermic reaction = the term exothermic process describes a process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light, electricity,

Endothermic reaction = An endothermic process is any process with an increase in the enthalpy H (or internal energy U) of the system. In such a process, a closed system usually absorbs thermal energy from its surroundings, which is heat transfer into the system.

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.