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 carbon's outer shell?

Two more hydrogen atoms will be required

Explanation:

A carbon atom has 4 valence electrons in its outermost shell,thus it can form 4 covalent bonds.Two pairs of electrons are shared in a double bond between C-C atom.two more electrons are left which is shared with two hydrogen atoms.