1.5 Nitrogen and Oxygen

Oxygen has a valence of 2 (it is divalent), and nitrogen's valence is usually 3 (trivalent). Oxygen and trivalent nitrogen are tetrahedral, like carbon. Here are the structures of ammonia (NH₃) and water (H₂O). The bonds are not exactly 109° apart because the hydrogen atoms repel each other slightly, but we can see that the tetrahedral geometry of the atoms is there.

The unbonded sites on the nitrogen and oxygen tetrahedra aren't just empty space; later on we'll see that there is something there and that it can participate in some types of non-covalent bonds.

In case you're wondering about the colors, they're strictly symbolic; atoms themselves don't have colors and can't be directly seen because they're much smaller than the wavelengths of visible light. These colors are part of a system called the CPK Standard, named after the initials of its creators. The very first ball and stick models color-coded their elements: black for carbon, by analogy with graphite and coal; red for oxygen, representing blood and fire; blue for nitrogen, representing blue sky and asphyxia; etc. These colors have been kept unchanged all this time and are still pretty much the only system in use today.

(I've always thought blue would be better for oxygen, since liquid oxygen itself is blue, and so is water, while nitrogen would make sense as red, since some of its oxides as well as azo compounds are red or yellow. But for these lessons I am sticking to the usual way of doing things.)

Next Lesson | Table of Contents | Main