Hydroxide Ion (OH−)
Hydroxyl Radical (OH)
Negatively Charged Ions
Alkaline Base

Hydroxide ion (OH−) is a stable, negatively charged ion, while a hydroxyl radical (∙OH) is a highly reactive, neutral free radical with an unpaired electron.

Chemical Formula and Charge:

Hydroxide ion has the formula OH−, indicating a single negative charge.

Hydroxyl radical has the formula ∙OH (or just OH), is electrically neutral, and features an unpaired electron (denoted by the dot).

Reactivity and Stability:

Hydroxide ions are relatively stable in solution and act as a strong base or a nucleophile in chemical reactions.

Hydroxyl radicals are extremely reactive and short-lived (lasting seconds) due to their unpaired electron. They aggressively "steal" electrons or hydrogen atoms from other molecules to achieve stability, causing damage to the targeted substance.

Role in Chemistry and Biology:

Hydroxide ions are a natural part of water due to self-ionization and are crucial in many general chemical processes, such as in alkaline solutions and the breakdown of organic molecules in digestion (hydrolysis).

Hydroxyl radicals are known as the "detergent of the atmosphere" because they oxidize and break down many pollutants. In biological systems, they are highly damaging and toxic, causing oxidative stress that can damage DNA, proteins, and cell membranes, and are linked to aging and disease.

The single electron difference in the hydroxyl radical is what makes it a potent and indiscriminate oxidizer, while the full electron shell of the hydroxide ion allows it to perform more controlled, fundamental chemical functions as an ion.