What is the highly reactive family of elements with 7 valence electrons?
Those are the halogens.
The halogens -- fluorine, chlorine, bromine, iodine, astantine, and the recently made element 117 -- are known also as Group 17 of the periodic table. They can access a stable noble gas electron configuration by adding one more electron.
They can do that by adding one electron from another atom to form a negative ion, like the chloride ions in sodium chloride. Or, they can share an electron with a partner atom, in exchange for the partner atom contributing a second electron, like a hydrogen atom pairing up with an iodine atom in hydrogen iodide. That is called a covalent bond. Either way, the ability to obtain a stable electron configuration with only one added electron makes halogens atoms easily pick up that electron.