Is CO Polar or Nonpolar?

Answer: CO is a polar molecule due to the large electronegativity difference between oxygen (3.44) and carbon (2.55) within the linear structure. Oxygen is left with a partial negative charge and carbon is left with a partial positive charge. 

Although the molecule is considered to be polar the necessary distribution of charge throughout the molecule does not match with the prediction. Since carbon is triple-bonded to oxygen, carbon requires a lone pair of electrons in order to complete its valence shell. When you therefore count the formal charges - carbon "possesses" five valence electrons and has a charge of -1. Conversely oxygen also "possesses" five valence electrons and has a charge of +1. This runs against the prediction made by electronegativity but this is a consequence of the necessary bonding structure. Therefore the polarity caused by electronegativity runs against distribution of charged electrons. As a result, the molecule shows a far weaker polar dipole. There are two consequences of this: 1.) carbon monoxide has a low melting and boiling point which are both near -200˚C and 2.) the structure is unstable and only lasts for short periods of time when it naturally appears in the atmosphere.

To learn more about the more stable yet related molecule Carbon Dioxide (CO2), feel free to check out the following articles regarding the polarity and the Lewis Dot Structure for CO2.

CO Ball and Stick Model
CO Ball and Stick Model. Created with Avogadro.
How is carbon monoxide (CO) utilized in the real world?

Although CO in quantities greater than 35 ppm is toxic since it disrupts normal hemoglobin function, carbon monoxide has been shown to play a role within the human body as a signaling molecule; for example, serving as a neurotransmitter or blood vessel relaxant. As a result of these properties, CO has been investigated in medical research studies as a possible anti-inflammatory agent. Carbon monoxide also finds use in chemical manufacturing and metallurgy (i.e. reducing metals from ores) due to its status as the simplest oxocarbon (i.e. the simplest combination of carbon and oxygen possible). 

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