An atom must be pruned if its subvalence equals zero. None of the bonds to such an atom can be promoted without altering the atom's charge attribute. Pruning the atom ensures the stability of its charge attribute, without interfering with bond promotion elsewhere. Viewed from another perspective, an atom with zero subvalence lacks unpaired electrons -at least within the narrow boundaries of the Balsa valence model. Such atoms can only form double bonds through changes to atomic charge.