The electrochemical reduction of atmospheric CO2 by renewable electricity opens new routes to synthesize fuels and chemicals, but more selective and efficient catalysts are needed. Herein, by combining experimental and first principles studies, we explain why chalcogen-modified copper catalysts are selective towards formate as the only carbon product. On the unmodified copper, adsorbed CO2 is the key intermediate, yielding carbon monoxide and formate as carbon products. On the sulfur- and selenium-modified copper, the chalcogen adatoms are present on the surface and actively participate in the reaction, either by transferring a hydride or by tethering CO2 thus suppressing the formation of CO. These results highlight the active role of chalcogen centers via chemical steps, and point towards basicity as the key descriptor for the stability and selectivity of these catalysts.