The electronic affinity is equal to the released energy when one adds a electron to an atom (or a molecular structure) taken (E) in a gas state:

The electronic affinity is measured in eV or Joule.

The addition of one electron to the atom can give a reaction Exothermique or Endothermique. If the reaction is with being exothermic, the atom releases from energy to receive this electron, it is as if it had " the affinité" for this electron, the electronic affinity will be positive. That occurs when the addition of an electron is favorable to the atom, by filling a layer for example. If the Réaction is with being endothermic, the electronic affinity will be then negative. That occurs when the addition of an electron is unfavourable with the atom, for example if a layer is already filled and that an electron is added, the atom will be found then with an additional layer, it will be necessary then to force in providing much energy to give to the atom an electron, to provide positive energy to the reaction it is to release from negative energy by the reaction.

The opposite concept (energy concerned for to tear off an electron with an atom) is the energy of ionization.

The Halogène S, which have at the same time a strong energy of ionization and the possibility of receiving an electron on same layer N have a great electronic affinity. On the other hand, rare gases have a positive electronic affinity: the additional electron is pushed back by the atom. Indeed, it should be placed on the layer (n+1), very far from the core, and strongly pushed back by the screen of the other electrons.

The rare Gas S have an almost null electronic affinity and extremely high potentials of ionization. Consequently, under normal conditions, they do not gain nor do not lose electrons and do not form connections: the only forces which bind the atoms between them are Forces of Van der Waals.