Electron affinity (EA) is defined as the change in energy when an electron is added to an atom to form an anion. The first EA corresponds to adding one electron to an atom:
Many elements have negative EA1, which means that the energy of the 1− anion is lower than the energy of the neutral atom plus the free electron; that is, the anion is more stable.
For other elements, EA1 is positive, meaning that the anion is less stable compared to the neutral atom plus a free electron. For these elements, an input of energy is required to form the anion, and, in the gas phase, the anion readily dissociates to yield the neutral atom and a free electron because the latter are lower in energy.