Fluorine (F 2,7)
has seven outer electrons, arranged in three lone pairs and one
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