Fluoride

Quick Facts

Formula: F⁻

Pronunciation: FLOO-ride

Etymology: Fluorine + -ide

Meaning: The negative ion formed when fluorine gains one electron.

What It Really Means

Fluoride is the smallest and most electronegative anion in the periodic table. That small size matters. A fluoride ion is tiny, with a concentrated negative charge. That makes it very good at grabbing protons and very good at attacking silicon. That is why fluoride is used in toothpaste to harden tooth enamel. It replaces hydroxide in the hydroxyapatite structure and forms fluorapatite, which is more resistant to acid attack from bacteria. Now here is a common confusion. Fluoride is not the same as fluorine. Fluorine is the element, a pale yellow gas that is extremely reactive. Fluoride is the ion, F minus, which is stable and safe in small amounts. The gas is dangerous. The ion in toothpaste is not. Same element, very different form. Also, watch the difference between fluoride and fluorite. Fluoride is the ion. Fluorite is the mineral, calcium fluoride, CaF2. And there is a subtle exam trap. Fluoride is a strong base but a weak nucleophile in water, because water molecules surround it tightly and block its path. That is a classic example of the difference between basicity and nucleophilicity. Small size helps basicity but hurts nucleophilicity in protic solvents.

Common Examples

• Sodium fluoride (NaF)
• Calcium fluoride (CaF₂)
• Hydrofluoric acid (HF)
• Fluorapatite

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