Sodium hydroxide is composed of sodium (Na) and hydroxide (OH) ions. The bonding between these ions is ionic. In an ionic bond, electrons are transferred from one atom to another, creating a positive and a negative ion. In the case of sodium hydroxide, the sodium atom loses one electron to the oxygen atom in the hydroxide ion. This creates a sodium ion (Na+) and a hydroxide ion (OH-). The sodium and hydroxide ions are then held together by electrostatic forces.

The ionic bonding in sodium hydroxide is responsible for its many properties, such as its high melting point, solubility in water, and ability to conduct electricity.