The distribution of ionized, molecular, and atomic hydrogen in the disk and especially along the spiral arms of the Whirlpool galaxy, M51, is investigated by combining recently published maps of the H-alpha, H2, and H I surface densities, and a twofold result is reported. The existence is established of a symmetric pattern of peaks and dips in the arm/interarm ratio of the massive star formation efficiency along the two main arms, similar to the patterns previously found in NGC 3992 and NGC 628. The pattern is due to enhanced on-arm star formation at specific radii in the disk. They correspond to antiresonances of a density wave system, suggesting that the resonance pattern locally affects the process of star formation. The high peaks in the H-alpha distribution along the arms are not reproduced in H2 - by far the most abundant form of neutral hydrogen in the inner disk. It is argued that the peaks do show up, although less strongly in the H I surface density along the arms.