Halogens
Halogens form group 17: fluorine, chlorine, bromine, iodine, astatine, tennessine. With seven valence electrons, they readily gain one more to reach the noble gas configuration — they are the most electronegative non-metals.
Fluorine is the most electronegative of all elements (Pauling = 3.98) and one of the most reactive known: it attacks even glass and water. Chlorine, more common, is massively produced for organic chemistry (PVC, solvents) and water sanitation.
Their chemistry is dominated by the −1 state (halides) and high positive states (+3, +5, +7 for Cl, Br, I in oxoanions). Hydrogen halides (HF, HCl, HBr, HI) are strong acids — except HF, which is weak because of the very short H–F bond.
Applications: toothpaste and drinking water (F), disinfection (Cl), antiseptic and thyroid nutrition (I), flame retardants (Br). Astatine and tennessine are synthetic elements with extremely short lifetimes.