Chapter 9 - Periodic Properties

Chapter 9 - Periodic Properties - PERIODIC PROPERTIES...

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50 PERIODIC PROPERTIES Ionization Energy ... IE Atoms exhibit a photoelectric effect photoelectron spectroscopy A gaseous atom can absorb a photon (of sufficiently high energy) to eject an electron - ionization h ν = E photon = IE + m electron v 2 /2 eg. IE of the He atom its ground state (normally IE of an atom or ion refers to its ground state unless otherwise specified) Ionization energy - the energy required to eject the electron eg. n = 1 in case of H atom in its ground state kinetic energy of the ejected electron E > 0 ----------- KE of ejected e E = 0 ----------- h ν IE E 1s ----------- IE(He) = E E 1 = 0 ( 2373 kJ/mol) = 2373 kJ/mol vs. IE(H) = 1312 kJ/mol
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51 He(g) He + (g) + e (g) E = IE(He) = 2373 kJ/mol vs. H(g) H + (g) + e (g) E = IE(H) = 1312 kJ/mol IE(He) > IE(H) the electrons in He are more tightly held in He - due to greater nuclear charge Li has two " first ionization energies " (note - the second ionization energy refers to Li + Li 2 + + e ). when Li(g) is subjected to sufficiently high frequency light, faster & slower electrons are emitted ... h ν h ν h ν E > 0 ------------------ KE of ejected e h ν E > 0 ------------------ KE of ejected e E --------------------- IE 1 E 2s --------------------- h ν IE 1 ΄ E 1s ---------------------
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52 The lower of these "first ionizations" is quoted as the ionization energy - it gives the lowest energy photon that will ionize the atom. The observation of two first ionizations energies is experimental verification of the electronic shell structure of the atom.
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