1. Ionization energy Energy needed to remove an electron from gaseous atom or ion in the ground state: X (g) → X + (g) + 1e - Endothermic process Li : 1s 2 2s 1 Li + : 1s 2 Highest energy electron removed 3+ 2e - 1e - Electron attracted to nucleus The higher the charge exerted by nucleus (effective nuclear charge, Z eff ) The closer the electron is to nucleus F larger More difficult to remove the electron Higher energy needed Larger ionization energy Energy must be supplied to move the electron away from nucleus (similar to throwing a stone upwards). II/

2. Z eff constant n ↑ r ↑ n constant Z eff ↑ r ↓ I.E. ↓ I.E. ↑ II/

3. Exceptions (irregularities) II/

4. Origin of these irregularities: → electron configuration Be: [He] 2s 2 B: [He] 2s 2 2p 1 Filled subshell/stable Difficult to destroy I.E. (group II A) > I.E. (group III A) p-subshell neither filled nor half-filled, electron easy to remove, reaching thereby filled subshell configuration N: [He] 2s 2 O: [He] 2s 2 Half-filled subshell/stable Difficult to destroy I.E. (group V A) > I.E. (group VI A) 2p neither filled nor half-filled, electron easy to remove, reaching thereby half-filled subshell configuration II/

5. Exercise: Arrange the elements in the expected order of increasing ionization energy: a) Mg, S, Si Mg < Si < S b) As, N, P As < P < N c) As, Ge, P Ge < As < P d) Mg, Al, Ga Ga < Al < Mg II/

6. Higher ionization energies: B → B + + e - I.E. = 801 kJ/molfirst I.E. B + → B 2+ + e - I.E. = 2427 kJ/molsecond I.E. B 2+ → B 3+ + e - I.E. = 3660 kJ/molthird I.E. B 3+ → B 4+ + e - I.E. = 25025 kJ/molfourth I.E. B 4+ → B 5+ + e - I.E. = 32822 kJ/molfifth I.E.  The higher the charge of the parent species, the larger is I.E. More difficult to remove an electron due to increased attraction forces. Attraction between B + and e - are weaker than attraction between B 2+ and e -.  big jump in I.E. when core electrons start to be removed: Very difficult to remove an electron from a closed, completely filled main shell !! B: [He] 2s 2 2p 1 II/

7.  big jump in I.E. when core electrons start to be removed:  electrons from a lower main shell start to get removed. II/

8. Exercise: To what main group does the element X belong? X → X + + e - I.E. = 1012 kJ/mol X + → X 2+ + e - I.E. = 1896 kJ/mol X 2+ → X 3+ + e - I.E. = 2910 kJ/mol X 3+ → X 4+ + e - I.E. = 4954 kJ/mol X 4+ → X 5+ + e - I.E. = 6272 kJ/mol X 5+ → X 6+ + e - I.E. = 21270 kJ/mol X 6+ → X 7+ + e - I.E. = 25410 kJ/mol X 7+ → X 8+ + e - I.E. = 29840 kJ/mol We ask ourselves: where is the big jump in I.E.? valence electrons core electrons Five valence electrons: fifth group, V A II/