Carbon has the valency of 4 which is also known as the tetravalency of the carbon atom for carbon it is very hard to lose all its electrons and to gain all the electrons from other elements this is the reason that carbon makes covalent bond while Oxygen have the valency of 2 which makes it to gain 2 electrons.
In the carbon dioxide molecule there is two molecule of Oxygen over one carbon which valence it's outermost shell and make the compound called carbon dioxide.
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