Carbon (C 2,4) and the elements that follow all are nonmetals. Pure carbon occurs both as the slightly metallic black graphite, and the clear and extremely hard nonmetallic diamond. But carbon is much more familiar in the myriad compounds that it forms with other elements. Carbon compounds are the basis of all life. We cannot open our eyes on this planet without seeing carbon compounds, from the most distant treeline to the tip of our own nose.

Nitrogen (N 2,5) has five outer electrons. It combines readily

with carbon to form compounds in living organisms. Nitrogen is found as two-atom or diatomic molecules, (recall the molecules of hydrogen). gas makes up nearly 80% of the Earth's atmosphere. The electron-shell diagram for nitrogen above, and the Lewis dot structure on page 7, both suggest that two of the five outer electrons are paired in some way. This is true. The pairing of electrons around an atom is as important in determining its chemical properties as is the pairing of electrons in a bond between atoms.