The Decrease In IE1 From Be to B Primarily is Due

TP The decrease in IE1 from Be to B primarily is due to … 20% 20% 20% 20% 20% 1. 2. 3. 4. 5. increase in atom size increase in the number of loops in the atomic orbitals increase in electrical shielding increase in effective nuclear charge some other reason The goal: Why 1s22s and not 1s22p 1s2 portion of Li electron cloud The Li atom electron configuration 1s22s is more stable than 1s22p. The reason is, the 2s electron feels a greater nuclear charge, Zeff, than does the 2p electron. The following illustrations http://goo.gl/hMNPLA show qualitatively why this is so. 1 2s portion of Li 1s22s electron cloud 2s outer loop of Li 1s22s electron cloud 2s outer loop is shielded by 1s2 cloud For ~90% of 2s, Zeff,outer ≈ 0.90 x (3 – 2) = 0.9 2 2s inner loop of Li 1s22s electron cloud 2s inner loop is not shielded by 1s2 cloud For ~10% of 2s, Zeff,inner ≈ 0.10 x 3 = 0.3 Zeff,2s = Zeff,outer + Zeff,inner = 0.9 + 0.3 = 1.2 3 Li 1s22s valence electron energy E2s 13.6 eV Zeff,2s2/4 13.6eV x 1.22/4 2p portion of Li 1s22p electron cloud −4.90 eV 2p is shielded by 1s2 cloud For ~100% of 2p, Zeff,2p ≈ 1 x (3 – 2) = 1 4 Li 1s22p valence electron energy Li 1s22s more stable than Li 1s22p E2p E2s 13.6 eV Zeff,2p2/4 13.6 eV x 12/4 3.40 eV 13.6 eV Zeff,2s2/4 13.6eV x 1.22/4 E2p −4.90 eV 13.6 eV Zeff,2p2/4 13.6 eV x 12/4 3.40 eV The reason is, the 2s electron feels a greater nuclear charge, Zeff, than does the 2p electron. Building electron configurations IE1 = 13.6 eV Zeff2/n2 Make a sketch of IE versus atom, for H through Na. The goal: Understand the pattern of stability across the periodic table. 1 6 5 3 4 2 7 5 Li 1s3? Energy order of relative spins (Pauli) Not possible, since at least two electrons would have the same spin in the same orbital. Key point: “spin” Such an electron wave vanishes everywhere and so there can be no atom with this configuration. Li 1s22s or 1s22p? ↑↑ impossible; ↑↓ worst ↑…↓ ↑ … ↑ best Be 2s2 or 2s2p? The 2s inner loop escapes the shielding of the 1s2 part of the electron cloud. The 2p has no inner loop and so would be more shielded by the electron cloud. magnetic moment, either up or down 1s2 part of the Hence, the 2s electron experiences slightly greater nuclear charge Zeff and so it is more tightly held than the 2p electron would be. 2s2 are in same orbital and so must have greater electron‐electron repulsion But, the 2s inner loops result in greater nuclear attraction than the 2p. Nuclear attraction trumps electron repulsion, and so 2s2 is more stable. So, 1s22s is more stable than 1s22p 6 B 2s3 or 2s22px or 2s22py or 2s22pz? C 2px2 or 2px2py? 2s3 is not possible for the same reason as 1s3 is not. 2px2 are in same orbital and so must have greater electron‐electron repulsion Either 2s22px or 2s22py or 2s22pz are possible and equivalent. What about electrons in different 2p orbitals? Energy order of relative spins (Pauli) C 2px2 or 2px2py? Key point: “spin” 2px2 are in same orbital and so must have greater electron‐electron repulsion magnetic moment, either up or down ↑↑ impossible; ↑↓ worst ↑…↓ ↑ … ↑ best ↑↓ worst : Electrons in same spatial region orbital and so repel one another the most ↑ … ↓: Electrons clump together Fermi clump in different spatial regions orbitals and so repel somewhat less What about electrons in different 2p orbitals? 2px2py can have spins parallel and so decreased repulsion Fermi hole . Both configurations have the same nuclear attraction Zeff and no inner loops . Hence 2px2py or 2px2pz or 2py2pz more stable. ↑ … ↑ best : Electrons “avoid” one another Fermi hole and are in different spatial regions orbitals and so repel one another the least 7 N 2px22py or 2px2py2pz? O 2px2py22pz or 2px2py2pz3s? In 2px22py the 2px is in same orbital and so must have greater electron‐electron repulsion 2px2py22pz has increased electron repulsion Fermi clump . In 2px2py2pz all spins are parallel and so there are only Fermi holes and so reduced repulsion. Hence 2px2py2pz is more stable. 2px2py2pz3s has decreased electron repulsion Fermi hole . 2px2py22pz has greater nuclear attraction since the n bound than n 3 orbitals. 2 orbitals are more Nuclear attraction trumps electron repulsion, and so 2px2py22pz is more stable. 8