Suppose that the carbon atom pictured here as a reactant had already formed a double bond with another carbon atom. how many hydrogen atoms would now be required to fill the carbons outer shell? bond formation between carbon and hydrogen to form methane. suppose that the carbon atom pictured here as a reactant had already formed a double bond with another carbon atom. how many hydrogen atoms would now be required to fill the carbons outer shell? bond formation between carbon and hydrogen to form methane. one two three four