Which of the following represents the hydroxide ion?

The hydroxide ion is represented by the chemical formula OH-. This is a polyatomic ion consisting of one oxygen atom covalently bonded to one hydrogen atom. The negative charge indicates that it has gained an extra electron, making it a negatively charged ion. Hydroxide ions play an essential role in various chemical reactions, especially in acid-base chemistry, where they often act as a base by accepting protons (H+ ions).

Understanding the hydroxide ion's structure and charge is crucial, as it significantly impacts pH levels in solutions and contributes to the basicity of substances. In contrast, the other options represent different species: H3O+ is the hydronium ion, H+ is a proton, and Na+ is a sodium ion, none of which fulfill the criteria of being a hydroxide ion. Therefore, the representation of the hydroxide ion by OH- is accurate and underscores its fundamental role in chemistry, particularly in aqueous solutions.