The halogens are chemistry's most electron-hungry elements. With seven electrons in their outer shell, they need just one more to achieve a noble gas configuration, and they pursue it with ferocious reactivity. Fluorine, the lightest halogen, is the most reactive element of any kind on the entire periodic table — it attacks almost every substance, including glass, and even reacts with some noble gases. The name "halogen" comes from the Greek for "salt-former," and it fits perfectly: combine any halogen with a metal and you get a salt. Sodium chloride (table salt), potassium iodide (added to salt for thyroid health), and calcium fluoride (the mineral fluorite) are all halogen salts.
What makes halogens uniquely instructive is that they demonstrate every state of matter at room temperature within a single group. Fluorine and chlorine are gases — fluorine is pale yellow, chlorine is yellow-green. Bromine is one of only two elements that are liquid at room temperature (mercury is the other), with a dark reddish-brown color and choking vapors. Iodine is a gray-purple solid that sublimes easily, producing violet fumes. Astatine, the rarest naturally occurring element, is so scarce and radioactive that its physical properties have never been directly observed — at most 25 grams exist in the entire Earth's crust at any moment.
Halogens are indispensable in daily life. Chlorine disinfects drinking water for billions of people, fluoride strengthens tooth enamel, and iodine is essential for thyroid hormones that regulate metabolism. Bromine compounds serve as flame retardants in electronics. Hydrofluoric acid etches glass and is used in semiconductor manufacturing. Teflon (PTFE) — the nonstick coating on frying pans — is a fluorine-containing polymer. Even photography historically depended on silver halide crystals that darken when exposed to light.