What is the chemical formula for the hydroxide ion?

The hydroxide ion is represented by the chemical formula OH-. This ion consists of one oxygen atom and one hydrogen atom, and it carries a negative charge, denoted by the "-" sign. Hydroxide ions are commonly found in bases and play a significant role in acid-base chemistry, where they can react with hydrogen ions (H+) to form water (H2O).

Other options present different concepts that do not represent the hydroxide ion. H2O is the formula for water, which contains two hydrogen atoms and one oxygen atom, but no charge. H3O+ is the hydronium ion, which occurs when water molecules combine with extra hydrogen ions, representing an acidic environment. NaOH is sodium hydroxide, a compound that includes the hydroxide ion but is not the ion itself. Thus, OH- is the correct representation of the hydroxide ion.