fluoride

Negative ion formed when hydrogen fluoride dissolves in water; compound formed between fluorine and another element.

The ending ‘ide’ in halides (such as fluorides, chlorides, and iodides) indicates that the halogen acts as the more electronegative partner (see electronegativity). As fluorine is the most electronegative element of all, this would apply to all of its compounds, but fluorinated hydrocarbons are not normally referred to as fluorides.

In parts of India, the natural level of fluoride in water is 10 parts per million. This causes fluorosis, or chronic fluoride poisoning, mottling teeth and deforming bones.