The free-valence index Fr for carbon atom r in a planar conjugated compound is defined as Fr ≡ √3 - ∑is prs, where the sum is over the atoms bonded to atom r. [The quantity 23 is the value of ∑is prs, for the central atom of the diradical C(CH2)3, whose central C has the largest possible value for this sum of any trigonally bonded carbon.] Fr is a measure of the unused bonding power of atom r and is used to estimate the susceptibility of a given atom to attack by an uncharged free radical. Calculate Fr for each carbon in 1, 3-butadiene, and state which carbon is predicted to be preferentially attacked by free radicals.