which lowers its energy → Excess energy released. Electron affinity Energy released or absorbed when an electron is added to a gaseous atom or ion in the ground state: X(g) + 1e- → X-(g) usually exothermic process If removing an electron is endothermic, then the reversed process is exothermic. An isolated electron is brought from far away to undergo attraction to the nucleus which lowers its energy → Excess energy released. 7e- 2e- e- 9+ F(g) + 1e- → F-(g) DH=-328 kJ/mol II/

Second E.A always endothermic: O-(g) + 1e- → O2-(g) DH > 0 II/ Electron attracted to nucleus F (attraction) larger Potential energy of electron and atom strongly reduced Released energy larger More negative electron affinity The higher the charge exerted by nucleus (effective nuclear charge, Zeff) The closer the electron is to nucleus (smaller r) F(g) + 1e- O(g) + 1e- -141 kJ/mol O-(g) Epotential -328 kJ/mol F-(g) Second E.A always endothermic: O-(g) + 1e- → O2-(g) DH > 0 II/

r ↓ n ↑ r ↑ EA more negative n constant Zeff ↑ Zeff constant (3)-(12) 3A (13) 4A (14) 5A (15) 6A (16) 7A (17) 8A (18) H -73 He >0 Li -60 Be 0 B -27 C -122 N 0 O -141 F -328 Ne >0 Na -53 Mg 0 Al -43 Si -134 P -72 S -200 Cl -349 Ar >0 K -48 Ca 0 Ga -30 Ge -120 As -78 Se -195 Br -325 Kr >0 Rb -47 Sr 0 In -30 Sn -120 Sb -103 Te -190 I -295 Xe >0 Cs -46 Ba 0 Tl -20 Pb -35 Bi -91 Po -180 At -270 Rn >0 Zeff constant n ↑ r ↑ EA less negative A positive electron affinity (EA) means a resistance to gaining another electron. The more negative the EA, the more strongly the atom attracts another electron. (F with an EA of -328 kJ/mol attracts electrons more strongly than O which has an EA of -141 kJ/mol). II/

II/ Halogens (group 7A, F to At) Most negative EA values, addition of an e- leads to noble gas configuration, very favorable. Group 5A (N to Bi) ½ filled shell discourages addition of an electron, EA values less negative than neighbors (groups 4A & 6A). Alkaline Earths (group 2A, Be to Ba) Filled s subshell discourages addition of an electron, EA values nearly zero. Noble Gases (Group 8A, He to Rn) Completely filled shell strongly discourages addition of an electron, EA values are positive. 3rd period more negative than 2nd period atoms are larger, more space for the electrons, repulsion between electrons less, more favorable 7e- 8e- 2e- 7e- 2e- 17+ 9+ II/ EA (Cl)=-348 kJ/mol EA (F)=-328 kJ/mol