Iodine has seven valence electrons. A valence electron is an electron in the outer shell of an atom that is able to participate in bonding. While iodine has 53 electrons, all but seven are in the inner shells of the iodine atom.
In chemistry, an atom has a full shell if it has eight valence electrons. Atoms with full shells are stable and are essentially inert. They will not react with other atoms to form bonds. Iodine is one valence electron short of having a full shell and will readily form bonds with other atoms to fill the missing space. Like iodine, the other atoms in the halogen family also have seven valence electrons.