Fluorine is a highly reactive, pale yellow gas at room temperature and is the most electronegative and reactive of all the elements. It is the lightest halogen and exists as a diatomic molecule, F2, in its free state.

Physical Characteristics

Symbol: F
Atomic Number: 9
Atomic Weight: Approximately 18.998403163
State at Room Temperature: Gas
Color: Pale yellow
Density: About 1.696 g/L at 0°C and 1 atm
Melting Point: -219.67°C (-363.41°F)

Applications

Manufacture of Fluorinated Compounds: Fluorine is used in the production of a wide range of fluorinated organic and inorganic compounds, including refrigerants, blowing agents, and polymers like polytetrafluoroethylene (PTFE), commonly known as Teflon.
Nuclear Industry: Fluorine is a key ingredient in the preparation of uranium hexafluoride (UF6) used in the uranium enrichment process, critical for producing fuel for nuclear reactors and nuclear weapons.
Dental and Medical Applications: Fluoride, a compound of fluorine, is added to dental products and water supplies to prevent dental cavities. Fluorine-18, a radioactive isotope, is used in positron emission tomography (PET) scans, a type of medical imaging.
Pharmaceuticals and Agrochemicals: Many pharmaceuticals and agrochemicals are fluorinated to enhance their activity or durability. Fluorine atoms can significantly alter the biological activity of these molecules.

Main Compounds

Hydrofluoric Acid (HF): A solution of hydrogen fluoride in water, hydrofluoric acid is used in the manufacture of fluorocarbons and as an etchant for glass and silicon in the semiconductor industry.
Sodium Fluoride (NaF): Used in toothpaste and water fluoridation to prevent dental cavities.
Uranium Hexafluoride (UF6): Used in the uranium enrichment process that produces fuel for nuclear reactors and weapons.
Sulfur Hexafluoride (SF6): A potent greenhouse gas used as an electrical insulator in high-voltage electrical equipment.

Basics on fluorine-Wikipedia