Final answer:
The process in which a molecule or atom gains electrons is called reduction, and when it results in a negatively charged ion, it's called an anion. The involved energy change is known as electron affinity. Even if reduction is energetically unfavorable, it can be part of favorable redox reactions.
Step-by-step explanation:
When a molecule or atom gains electrons, this process is called reduction. A neutral atom that gains an electron becomes negatively charged and is referred to as an anion. For example, chlorine will gain one electron to become Cl- or oxygen will gain two electrons to become O2-. The energy change accompanying an atom gaining an electron is known as electron affinity. Electron affinities are often measured in kilojoules per mole (kJ/mol) and represent the energy released when an atom in its gaseous state accepts an electron.
An energetically unfavorable process means that the reaction or process requires energy to proceed. This is often associated with the concept of endothermic reactions, where energy is absorbed rather than released. However, during redox reactions, the element gaining electrons (being reduced) might still be part of an overall energetically favorable process, depending on the wider context of the reaction.
A chemical reaction involving changes in oxidation numbers is termed a redox reaction, with the gaining of electrons termed reduction, which leads to a decrease in the oxidation number of the atom or molecule.