Building Electron Configurations Lecture Note

TP The steady increase in IE1 from Li to Ne 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 Response Counter Building electron configurations 10 1 IE1 = 13.6 eV Zeff2/n2 The goal: Understand the pattern of stability across the periodic table. Make a sketch of 1 versus atom, for H through Na. 6 5 3 4 2 5 7 6 1 TP The steady increase in IE1 from Li to Ne primarily is due to … 0% 0% 0% 0% 0% 1. 2. 3. 4. 5. Electrons have a magnetic moment 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 Stern–Gerlach experiment: Silver atoms travelling through an inhomogeneous magnetic field, and being deflected up or down depending on their spin: 1 furnace, 2 beam of silver atoms, 3 inhomogeneous magnetic field, 4 classically expected result, 5 observed result. 7 12 Electrons have a magnetic moment Relative direction of spins (Pauli principle) Was thought to be due to the electron spinning on its axis. The magnetic moment can only be oriented “up” ↑ or “down” ↓ . We now know this is not the case. This means spins of the two electrons can either be parallel ↑↑ or ↓↓ or antiparallel ↑↓ Rather it is instead an intrinsic property of the electron. Nonetheless, the magnetic moment is referred to as the “spin” of the electron. It turns out that if electrons are in the same orbital, only ↑↓ is possible. This means no more than two electrons can be in the same orbital, because more than two would require two electron spin to be parallel, say ↑↓↑. This restriction to no more than two electrons in an orbital is known as the Pauli Principle. 13 14 2 Li 1s3? Li 1s22s or 1s22p? Not possible, since at least two electrons would have the same spin in the same electron cloud. The 2s inner loop escapes the shielding of the 1s2 part of the electron cloud. Such an electron wave vanishes everywhere and so there can be no atom with this configuration. The 2p has no inner loop and so would be more shielded by the 1s2 part of the electron cloud. Hence, the 2s electron experiences slightly greater nuclear charge Zeff and so it is more tightly held than the 2p electron would be. So, 1s22s is more stable than 1s22p 15 16 Be 2s2 or 2s2p? B 2s3 or 2s22px or 2s22py or 2s22pz? 2s2 are in same orbital and so must have greater electron‐electron repulsion 2s3 is not possible for the same reason as 1s3 is not Pauli . But, the 2s inner loops result in greater nuclear attraction than the 2p. Either 2s22px or 2s22py or 2s22pz are possible and equivalent degenerate . Better to have less repulsion 2s2p or greater attraction 2s2 ? Greater attraction more important than less electron repulsion, and so 2s2 is more stable. 17 18 3 TP In C 2px2py which spin arrangement is more stable? C 2px2 or 2px2py? 2px2 are in same orbital and so will have greater electron‐electron repulsion So the atom is more stable if the electrons are different 2p orbitals. 33% 1. ↑ … ↓ magnetic moments opposite, N to S 33% 2. ↑ … ↑ magnetic moments the same, N to N 33% 3. The arrangement are equally stable What about the relative spins? 19 20 C 2px2py C 2px2 or 2px2py? It turns out that the arrangement ↑ … ↑ results in the two electrons avoiding each other and so repelling a little less. This avoidance is call a Fermi hole and is always present with parallel spins. 2px2 are in same orbital and so must have greater electron‐electron repulsion On the other hand the arrangement ↑ … ↓ results in the two electrons encountering each another and so repelling little more. This encountering is called a Fermi heap and is always present with antiparallel spins. 2px2py has less electron‐electron repulsion 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 with spins ↑ … ↑ is most stable. For this reason, C atoms are paramagnetic attracted to a magnet . 21 23 4 N 2px22py or 2px2py2pz? N 2px22py or 2px2py2pz? In 2px22py the 2px is in same orbital and so must have greater electron‐electron repulsion In 2px2py2pz all spins are parallel and so there are only Fermi holes and so reduced repulsion. Hence 2px2py2pz with spins ↑ … ↑ … ↑ is more stable. 24 O 2px2py22pz or 2px2py2pz3s? 25 O 2px2py22pz or 2px2py2pz3s? 2px2py22pz has ↑ … ↑↓ … ↑ and so increased electron repulsion same orbital and Fermi clump . 2px2py2pz3s has ↑ … ↑ … ↑ … ↑ and so decreased electron repulsion different orbitals and Fermi hole . 2px2py22pz has greater nuclear attraction since the n bound than n 3 orbitals. 2 orbitals are more Nuclear attraction more important than electron repulsion, and so 2px2py22pz is more stable. 26 27 5 F 2px22py22pz or 2px2py22pz3s? F 2px22py22pz or 2px2py22pz3s? 2px22py22pz has ↑↓ … ↑↓ … ↑ and so increased electron repulsion same orbital and Fermi clump . 2px2py22pz3s has ↑ … ↑↓ … ↑ … ↑ and so decreased electron repulsion different orbitals and Fermi hole . 2px22py22pz has greater nuclear attraction since the n bound than n 3 orbitals. 2 orbitals are more Nuclear attraction more important than electron repulsion, and so 2px22py22pz is more stable. 28 Ne 2px22py22pz2 or 2px22py22pz3s? 29 Ne 2px22py22pz2 or 2px22py22pz3s? 2px22py22pz2 has larger eff, since 2p electrons do not shield 2px22py22pz3s has less electron repulsion but much smaller electron shield nearly completely. 30 eff, since 2 31 6 Na 2px22py22pz2 3s or 2px22py22pz2 3p? Na 2px22py22pz2 3s or 2px22py22pz2 3p? 3s inner loop means 3s has more attraction than 3p. Hence 2px22py22pz2 3s more stable analogous to 1s22s being more stable than 1s22p. 32 First ionization energy, IE1 33 Effective nuclear charge, Zeff Zeff is the value for which 1 energy formula, 6 1 1 6 5 5 3 3 4 2 is equal to the one‐electron 13.6 eV Zeff2/n2 1 7 4 7 2 34 35 7 TP The steady increase in 20% 20% 20% 20% 20% 1. 2. 3. 4. 5. 1 from Li to Ne primarily is due to … TP The decrease in 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 20% 20% 20% 20% 20% 36 Quiz The decrease in 20% 20% 20% 20% 20% 1. 2. 3. 4. 5. 1 1. 2. 3. 4. 5. 1 from Be to B primarily is due to … 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 37 from N to O primarily is due to … increase in atom size increase in the number of loops in the atomic orbitals increase in electrical shielding increase in effective nuclear charge increased electrical repulsion 38 8