Fluorine (F 2,7) has seven outer electrons, arranged in three lone pairs and one lone electron. is a yellow-green gas, and is even more reactive and heat-emitting when it combines with carbon compounds than oxygen is. But the evolutionary process still chose O and not  F for its energy-yielding chemistry, probably because F is scarce on this planet and not obtained easily.

Neon, with eight outer electrons (Ne 2,8) in four electron pairs, closes the series with a noble gas resembling helium.

Neon, like helium, does not combine chemically with anything; thus pure neon, like pure helium, exists as a monatomic gas.

This is as far as we can go with the first two electron shells. Any additional electrons would have to begin a third shell (which also can hold eight electrons) . We will return to this in Chapter 6, after we have looked more closely at the behavior of the second-shell elements.