In which of the following species is the central atom an exception to the octet rule?`NH_4^+` `I_2` `NH_3` `SF_6`

The octet rule is a heuristic, or "rule of thumb," about how electron configurations usually work in chemical bonds. It says that elements tend to gain or lose electrons until they have an octet, eight electrons, in their valence, or outermost, electron shell. Like any heuristic, it is usually, but not always, true. It has some important exceptions.`NH_4^+` is a positive ion of one nitrogen atom and four hydrogen atoms. Nitrogen has five electrons...

The octet rule is a heuristic, or "rule of thumb," about how electron configurations usually work in chemical bonds. It says that elements tend to gain or lose electrons until they have an octet, eight electrons, in their valence, or outermost, electron shell. Like any heuristic, it is usually, but not always, true. It has some important exceptions.

`NH_4^+` is a positive ion of one nitrogen atom and four hydrogen atoms. Nitrogen has five electrons in its valence shell, so we would expect it to take on three electrons to fill the octet. Hydrogen has only one valence electron, but the first shell only allows two electrons, so we expect it to accept or give one electron. Yet if each hydrogen atom gives one electron to the nitrogen atom, the nitrogen atom would now have nine electrons, not eight. However, since this is a positive ion, one of the electrons is actually missing, so the central nitrogen atom must actually only have eight electrons, which means it is not an exception to the octet rule.

`I_2` is a molecule formed between two iodine atoms. Iodine has 7 valence electrons, so we would expect each to take on one electron—but that can't happen. So this is an exception to the octet rule. (In fact what happens is the two atoms share two electrons in a covalent bond.)

`NH_3` is a molecule of one nitrogen atom and three hydrogen atoms. By similar reasoning with NH_4^+ we see that the three hydrogen atoms each give one electron to the nitrogen atom, totaling eight. Therefore this is not an exception to the octet rule.

Finally, `SF_6 ` is a molecule of one sulfur atom and six fluorine atoms. Sulfur has 6 valence electrons, while fluorine has 7. Fluorine is more electronegative, so we expect the fluorine atoms to each take one of the 6 electrons from the sulfur atom. This obeys the octet rule.

Therefore, the answer is (B) `I_2`. All the other molecules obey the octet rule, while this one does not because it is a covalent molecule.