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CHEM 1E03 (53)
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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 2 hν = E photon = IE + m electron/2 Ionization energy - the energy kinetic energy of the required to eject the electron ejected electron eg. n = 1 → ∞ in case of H atom in its ground state eg. IE of the He atom its ground state (normally IE of an atom or ion refers to its ground state unless otherwise specified) IE(He) = E ∞ − E 1 E > 0 ----------- = 0 − (−2373 kJ/mol) − KE of ejected e = 2373 kJ/mol E∞= 0 ----------- hν vs. IE E1s ----------- IE(H) = 1312 kJ/mol 50 hν + − He(g) → He (g) + e (g) ∆E = IE(He) = 2373 kJ/mol vs. hν + − 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 →iν Li2+ + e ). → when Li(g) is subjected to sufficiently high frequency light, faster & slower electrons are emitted ... E > 0 ------------------ KE of ejected e hν E′ > 0 ------------------ − KE′ of ejected e E∞--------------------- IE 1 E2s-------------------- hν ΄ IE1 E1s--------------------- 51 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. Electron Configuration & Ionization Energy: H: E I 1(H) = 1312 kJ/mol 2 He: E 2 1 1(He) = 2373 kJ/mol Li: 1s 2Es I 1(Li) = 520 kJ/mol → Li has a much lower first ionization energy → the 2s electron of Li is much more weakly held than the 1s electron of even H, in spite of the increased nuclear charge → the 2s orbital is an excited 1-electron state - it arises as an 2+ excited state of the one electron ion, Li → plus, the 2s electron is shielded from the nucleus by the two 1s electrons - the 2s electron "sees" something like + an Li core which has a "core charge" of 3 − 2 = 1 nuclear # of core charge electrons core charge = nuclear charge − (# of core electrons) = group number 52 ♣ 2 1 Li: 1s 2s I 1(Li) = 520 kJ/mol Be: 1s 2s E I 1(Be) = 899 kJ/mol 2 2 1 B: 1s 2s 2p 2 IE 1(B) = 801 kJ/mol C: 1s 2s 2p IE 1(C) = 1086 kJ/mol Be also has two first ionization energies - the lower value is the quoted first ionization energy (given above). → IE (B1) is greater than that of Li, because it is still a 2s electron being ejected - but this 2s electron "sees" a core charge of 4 − 2 = 2 - it is more tightly held than the valence electron of Li. 53 B has three first ionization energies: E > 0 ----------------------- E′ > 0 ----------------------- E′′ > 0 ----------------------- E ∞-------------------------- IE E 2p-----------IE 1--------- 1 E 2s------------------------ IE 1′ E 1s -------------------------- IE 1B) < IE (1e) even though B has a higher nuclear charge. This is because the highest energy valence electron of B is in a 2p orbital. → the 2p orbital is higher in energy than the 2s orbital (true for all atoms except H) → the 2p electron is partially screened from the nucleus by the 2s electrons, in addition to the 2p electrons C also shows three distinct (sub)shells in its photoelectron spectrum. IE (C) > IE (B) because the highest valence electron of C is in 1 1 2p, just like that of B. 54 Ionization Energy across the Periodic Table
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