▎ 摘　　要

The band gap of periodically doped graphene with hydrogen is investigated. It is found through a tight-binding (TB) model that for certain periodicities, called here nongap periodicities (NGPs), no gap is opened at the Dirac point. Density-functional-theory calculations show that a tiny gap is opened for NGPs due to exchange effects, not taken into account in the TB model. However, this tiny gap is one order of magnitude smaller than the gap opened for other periodicities different from NGPs. This remarkable reduction in the band-gap opening for NGPs provokes a crossing of the midgap and the conduction bands and, consequently, the metallization of the system. This result is also valid for other adsorbates different from atomic hydrogen.