Caption: An ABSTRACT Lewis structure diagram of the carbon monoxide (CO) molecule.

Features:

1. "Carbon monoxide (CO) is a colorless, odorless, tasteless flammable gas that is slightly less dense than air at standard temperature and pressure (STP). It is toxic to animals that use hemoglobin as a molecular oxygen (O_2) carrier (both invertebrate and vertebrate) when encountered in concentrations above about 35 parts per million (ppm)" (slightly edited from Wikipedia: Carbon monoxide).

2. As the image shows, carbon monoxide has a triple bond using 6 bonding electrons and each atom has 2 unbonded valence electrons (see Wikipedia: Carbon monoxide: Molecular properties). The bond length (i.e., the average center-to-center distanced of the atomic nuclei) is 112.8 picometers (pm) =1.128 angstroms (Å).

3. In the interstellar medium (ISM), carbon monoxide is the 2nd most abundant diatomic molecule after molecular hydrogen (H_2). Since molecular hydrogen (H_2) does NOT radiate strongly, carbon monoxide is an important tracer gas for molecular clouds. Note the interior of molecular clouds can only be observed in the radio band (fiducial range 3 Hz -- 300 GHz = 0.3 THz, 0.1 cm -- 10**5 km) and infrared band (fiducial range 0.7 μm -- 0.1 cm) because of the opacity of interstellar dust in the ultraviolet band (fiducial range 0.01--0.4 μm) and visible band (fiducial range 0.4--0.7 μm = 4000--7000 Å).

4. In fact, carbon monoxide (CO) is the dominant radiative line emission cooling molecule in molecular clouds despite being a trace species (Mark R. Krumholz, 2011, Physical Processes in Molecular Clouds). Molecular hydrogen (H_2) despite being overwhelming dominant in mass fraction is unimportant radiative line emission cooling.

5. It is NOT surprising that carbon monoxide is the 2nd most abundant diatomic molecule. Carbon (C) and oxygen (O) are the 2nd and 1st most abundant metals by mass fraction in the cosmic composition (see solar_composition.html which is a fiducial cosmic composition) and they are both reactive elements that easily form molecules---carbon is forms more compounds than any other element---it's "king of the elements" (Wikipedia: Carbon).