The first electron affinity is the enthalpy change accompanying the addition of one electron to each atom in one mole of gaseous atoms to form one mole of gaseous -1 ions under standard conditions. For exampleCl(g) + e- → Cl-(g)The first electron affinity is exothermic. The second electron affinity may be endothermic. This is because it is harder to add an electron to an already negative ion.