<P> Iodine is the fourth halogen , being a member of group 17 in the periodic table , below fluorine , chlorine , and bromine ; it is the heaviest stable member of its group . ( The scarce and fugitive fifth halogen , the radioactive astatine , is not well - studied due to its expense and inaccessibility in large quantities , but appears to show various unusual properties due to relativistic effects . ) Iodine has an electron configuration of ( Kr ) 4d 5s 5p , with the seven electrons in the fifth and outermost shell being its valence electrons . Like the other halogens , it is one electron short of a full octet and is hence a strong oxidising agent , reacting with many elements in order to complete its outer shell , although in keeping with periodic trends , it is the weakest oxidising agent among the stable halogens : it has the lowest electronegativity among them , just 2.66 on the Pauling scale ( compare fluorine , chlorine , and bromine at 3.98 , 3.16 , and 2.96 respectively ; astatine continues the trend with an electronegativity of 2.2 ) . Elemental iodine hence forms diatomic molecules with chemical formula I , where two iodine atoms share a pair of electrons in order to each achieve a stable octet for themselves ; at high temperatures , these diatomic molecules reversibly dissociate a pair of iodine atoms . Similarly , the iodide anion , I , is the strongest reducing agent among the stable halogens , being the most easily oxidised back to diatomic I. ( Astatine goes further , being indeed unstable as At and readily oxidised to At or At , although the existence of At is not settled . ) </P>
where is iodine found in the periodic table
the fourth halogen , being a member of group 17 in the periodic table , below fluorine , chlorine , and bromine