A positron is ejected from the parent nucleus, but the daughter (Z−1) atom still has Z atomic electrons from the parent, i.e. the daughter is a negative ion (at least immediately after the positron emission). Since tables of masses are for atomic masses, 
, and, since the mass of the positron is identical to that of the electron, the overall result is that the mass-energy of two electrons is required, and the β decay is energetically possible if and only if the mass of the parent atom exceeds the mass of the daughter atom by at least two electron masses (2m c = 1.022 MeV).
Isotopes which increase in mass under the conversion of a proton to a neutron, or which decrease in mass by less than 2m, cannot spontaneously decay by positron emission.