Electron Affinity Another property that influences an atom’s chemical behaviour is their ability to accept one or more electrons This property is called electron affinty electron affinty: the negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion X(g) + e X(g) Electron affinity is positive if the reaction is exothermic (releases energy) and negative if the reaction is endothermic (absorbs energy) F(g) + e F(g) H = 328 kJ/mol The electron affinity for fluorine is assigned as +328 kJ/mol. Ionization energy measures the ease with which an atom loses an electron, whereas electron affinity measures the ease with which an atom gains an electron. When the addition of an electron makes the atom more stable, energy is given off A large positive electron affinity means that the negative ion is very stable (that is, the atom has a great tendency to accept an electron) The halogens have the greatest electron affinity because they are one electron short of a filled p-block. When the addition of an electron makes the atom less stable, energy must be put in If the added electron must be placed into a higher energy level than the other valence electrons, then the element is made less stable Trend: Moving left to right across a period, the electron affinity increases (more energy given off becoming more stable) Properties of Metals Metals tend to have low ionization energies and therefore tend to form cations relatively easily Most metal oxides are ionic solids that are basic (metal oxide + water → metal hydroxide) Na2O(s) + H2O(l) → 2 NaOH(aq) Properties of Nonmetals Because of their large electron affinities, nonmetals tend to gain electrons when they react with metals Most nonmetal oxides are acidic (nonmetal oxide + water → acid) CO2(g) + H2O(g) → H2CO3(aq) .