Question

write the valency pair which have the similar formula​

Answer 1

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C is 4 and O is 2, so yes, it should be always CO2 and never CO. But this is not the case, CO2 is a covalent compound. But in CO, the electron pairs of Oxygen gets delocalized between C and O, creating a resonance structure.

So without another Oxygen atom, the structure becomes stable as Oxygen shares its lone pair with Carbon.

ʙᴇ ʏᴏᴜʀs............^_^