For many decades it has been assumed that an adsorbate centered above a metal surface and with a net negative charge should increase the work function of the surface. However, despite their electronegativity, N adatoms on Wf100g cause a significant work function decrease. Here we present a resolution of this anomaly. Using density functional theory, we demonstrate that while the N atom carries a negative charge, of overriding importance is a reduction in the surface overspill electron density into the vacuum, when that charge is engaged in bonding to the adatom. This novel interpretation is fundamentally important in the general understanding of work function changes induced by atomic adsorbates.