Would you expect fluorine or iodine to have a greater electronegativity? Why? (hint: It has to do with the nucleus.)

Answer 1

Fluorine has greater EN than iodine (in fact, F is the most EN atom in the periodic table). Fluoride has a high EN due to a combination of two effects, but the reason why iodine has a lower EN value is due to reason #2 alone.

Fluorine's high EN is due a combination of two effects:

1. The nearly complete valence shell of 7 electrons. All atoms want to seek to have a complete shell, to ensure that diamagnetic paired bonding of electrons. Fluorine seeks to complete it 2p valence shell by picking up an extra electron, making it very EN. Iodine experiences the same effect as fluorine, as it attempts to fill its outer-most valence shell (5p), to gain a noble gas configuration.
2. Lack of shielding due to few core electrons shells. The only inner electron shells for Fluorine are the 1s and 2s subshells, which provide little shielding from the positively charged nucleus. Thus, any nearby electrons are very attracted to the fluorine nucleus.

This effect differs for iodine. The iodine nucleus is buried beneath an additional 8 subshells (1s2s2p3s3p4s3d4p5s4d). So, as iodine tries to fill its outer most shell (5p), it experiences significantly more shielding and is significantly less attractive to nearby electrons.