Fluorine

chemistry · reactive elements · halogens

Super reactive gas. Fluorine is so eager to react with other elements that it will combine with almost anything except for the noble gases like helium and neon. Scientists have to be very careful when handling it because it reacts so violently.

Pale yellow color. Fluorine exists as a pale yellow gas at room temperature, and it's made up of pairs of atoms bonded together. This distinctive color helps scientists identify it in experiments.

Quite toxic. Elemental fluorine gas is highly poisonous to living things, so it must be stored safely in special containers. However, the fluorine compounds we use in toothpaste and refrigerators are totally safe.

Number 9 element. Fluorine has 9 protons in its nucleus and is the lightest of all the halogen elements, a group that includes chlorine and iodine. Being the lightest halogen makes it extra reactive.

Freezes at low temps. While it's a gas at room temperature, fluorine will liquefy if you cool it down enough, turning into a pale yellow liquid that boils away easily.