Here's why:

* Anions are negatively charged ions.

* Cations are positively charged ions.

The hydroxide ion has a net negative charge because:

* Oxygen has a higher electronegativity than hydrogen.

* This means the shared electrons in the bond spend more time closer to the oxygen atom.

* The oxygen atom gains a partial negative charge, and the hydrogen atom gains a partial positive charge.

* The hydrogen atom loses its electron and becomes a proton (H+), leaving the hydroxide ion with a net negative charge.