In considering the lewis structure of carbon monoxide, we must remember that the molecule contains a total of ten valence electrons. Four are the valence electrons that are present on the valence shell of carbon while six are the valence electrons on oxygen. Some of these valence electrons participate in bonding in the CO molecule.
Out of the six valence electrons on oxygen, two valence electrons participate in bonding with carbon while the other four electrons remain localized on the oxygen atom as two lone pairs of electrons.
Hence there are four nonbonding electrons in the lewis structure of CO as well as one triple bond.
