The halogen with the least negative electron affinity is astatine, which has an electron affinity of -270 kilojoules per mole. The lower electron affinity is related to the larger atomic size of astatine. In larger atoms, electrons will not be as attracted to the nucleus.
Astatine is part of the halogen group, which also includes fluorine, chlorine, bromine and iodine. It has an atomic symbol of At and an atomic number of 85. Halogens are more reactive than other elements due to the fact that they have seven valence electrons. They only need one more electron to have a full octet in their outer shell. Unlike the other halogens, astatine is radioactive.