The electron affinity is a measure of the energy change when an electron is added to a neutral atom to form a negative ion. For example, when a neutral chlorine atom in the gaseous form picks up an electron to form a Cl- ion, it releases an energy of 349 kJ/mol or 3.6 eV/atom. It is then said to have an electron affinity of -349 kJ/mol and this large number indicates that it forms a stable negative ion. Small numbers indicate that a less stable negative ion is formed. Groups VIA and VIIA in the periodic table have the largest electron affinities. Alkali earth elements (Group IIA) and noble gases (Group VIIIA) do not form stable negative ions. Periodic Trends: Electron Affinity

Remember: Electron Affinity is essentially the opposite of Ionization Energy. We are adding electrons to a ground atom- not removing them. Why are some elements at zero?

Remember: Electron Affinity is essentially the opposite of Ionization Energy. We are adding electrons to a ground atom- not removing them. Why are some elements at zero?

Periodic Trends: Electron Affinity Why isn’t Flourine the most accepting of the extra electron? The chart goes against the expected trend.

Why isn’t Flourine the highest Electron Affinity? Electron affinities generally become smaller down a column of the for two reasons. –the electron being added to the atom is placed in larger orbitals, where it spends less time near the nucleus of the atom- shielding –Second, the number of electrons on an atom increases as we go down a column, so the force of repulsion between the electron being added and the electrons already present on a neutral atom becomes larger. Remember: the repulsion between the electron being added to the atom and the electrons already present on the atom depends on the size of the atom. –In Groups VIA and VIIA, this repulsion is largest for the very smallest atoms in these columns: oxygen and fluorine. Thus, these elements have a smaller electron affinity than the elements below them.