What does reduction refer to in terms of electrons?

Reduction refers to the process in which an atom or molecule gains electrons. This increase in electron density typically results in a decrease in oxidation state, thereby making the species more negative or less positive. This concept is rooted in the definitions of oxidation and reduction, which are often taught using the mnemonic "LEO says GER," meaning "Lose Electrons, Oxidation; Gain Electrons, Reduction."

In the context of redox reactions, reduction always occurs simultaneously with oxidation, where one substance loses electrons (oxidation), while another substance gains those electrons (reduction). This duality is fundamental to understanding how chemical reactions involving transfer of electrons proceed, particularly in inorganic chemistry and electrochemistry. The focus on electron gain makes it clear that reduction is inherently linked to changes in the electron configuration of the reacting species, emphasizing its role in forming anions or less positively charged species.

The other choices relate to processes that do not match the definition of reduction; the loss of electrons describes oxidation, and proton transfer does not fit into the redox framework pertaining to electron transfer. Thus, the choice of gain of electrons accurately identifies the nature of reduction within chemical reactions.