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Atomic Electron Configurations and Chemical Periodicity

Atomic Electron Configurations and Chemical Periodicity. We know the electronic structure of the hydrogen atom states as determined by the quantum numbers n, l and m . How does this apply to larger atoms? i.e. multiple electron systems.

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Atomic Electron Configurations and Chemical Periodicity

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  1. Atomic Electron Configurations and Chemical Periodicity We know the electronic structure of the hydrogen atom states as determined by the quantum numbers n, l and m. How does this apply to larger atoms? i.e.multiple electron systems How does the electron structure relate to the periodic table? How does the electron structure relate to the chemical properties of atoms?

  2. Electron Spin and Magnetism Before we can talk about structure we need to learn a bit about the magnetic properties of particles. Recall that electron move the nucleus in orbits corresponding to set angular momentum values v Recall, also that when electrons move they generate a magnetic field, B. This is analogous to electrical current moving through a loop B

  3. v v B B Electron Spin and Magnetism Electrons in orbit generate magnetic fields, therefore all materials are magnetic. Is that so? Imagine two electrons in the same orbit moving in opposite directions. The magnetic fields cancel !! Do electrons occur in pairs in orbitals!!! Yes! But not for this reason, since this is not physically correct. Motion of electrons in their orbitals is not responsible for magnetism, even when the electron is unpaired. The net magnetic field averages to zero.

  4. Electron Spin and Magnetism When a beam of atomic hydrogen is passed through a non-uniform magnetic field is splits into two beams This Magnetism is not due to due to orbital motion Another source of magnetism From where? Spin

  5. Electron Spin and Magnetism B Magnetic field UP (s=1/2) DOWN (s=-1/2) Electron spin is an inherent magnetism associated with it, which has nothing to do with its translational motion. The electron can the thought of as a little magnet When an external magnetic field is applied the electron will either align along or against the field. Being aligned withthe field is more stable than against, therefore the up orientation is slightly favored The distribution of up to down depends on strength of the applied magnetic field. More stable Less stable

  6. Magnetic Materials DiamagneticMaterials Composed of atoms/molecules containing only paired electrons They are repelled by an externally applied magnetic field. ParamagneticMaterials Composed of atoms/molecules with unpaired electrons. More electron electrons will alignwith the field than against the externally applied field. The result is a net bulk magnetic field parallel to the applied field, hence an attractive force

  7. Magnetic Materials Ferromagnetic Materials – Have a permanent magnetic field When two electron on separate atoms are close, the field from one will cause the other to align with it as it would be more stable The magnetic field from each atom will add up, as long as the atoms are correctly aligned to give a one strong “bulk’ magnetic field. – ie.Magnets

  8. Pauli Exclusion Principle Fermions- particles have spin ½. electrons protons neutrons “Fermions cannot occupy the same space and spin coordinates” This means that no two electrons can have the same quantum numbers, including the spin quantum numbers. Therefore each orbital can only have 2 electrons since there are only two spin states s =1/2 and -1/2. Example: 1s orbital n = 1, l = 0, m = 0 and s = 1/2 or s = -1/2 1s Orbital

  9. Atoms with more than one electron Multi-electron wavefunctions are similar to those for the H atom The ground state of such atoms requires that the lowest possible energy wavefunctions be “occupied” box diagram - a simple tool used to add or subtract electrons from the boxes to represent the electron configuration of the element Consider H, He, Li and Be

  10. Hunds rule 1s 2s 2p Element # e’s B 5 Electrons added to each empty orbital in parallel C 6 N 7 When no new orbitals are available they are paired O 8 F 9 Maximize spin Ne 10

  11. Electron Configuration A shorthand notation is commonly used to write out the electron configuration of the atoms based on the number of electrons within each subshell It consists of: NUMBERLETTERSUPERSCRIPT (shell i.d.) (subshell) (occupancy)

  12. Electron Configuration 1s 2s 2p Element # e’s 1s2 2s2 2p1 B 5 1s2 2s2 2p2 C 6 1s2 2s2 2p3 N 7 1s2 2s2 2p4 O 8 1s2 2s2 2p5 F 9 1s2 2s2 2p6 Ne 10

  13. Aufbau order and Energy Levels The sequence of subshells in the electron configurations not the same as the energy levels of H The experimental sequence is known as the aufbau order It is a consequence of electron-electron interactions have on the energies of the wavefunctions in all multi-electron atoms Levels within subshells are still degenerate, the subshells in each shell are no longer degenerate in each shell, and differ in energy as s < p < d, Some subshells can overlap the levels of a differentshell; thus, for example, in neutral atoms 4s lies below 3d

  14. Traditional aufbau sequence diagram Instead of filling orbitals in order of increasing n, we should really be filling them in order of increasing n + l n is used as a ‘tiebreaker’ i.e the one with lowest n first Ex) Fluorine 9 e’s 2p5 1s2 2s2 Ex) Scandium 21 e’s 2p6 4s2 1s2 2s2 3s2 3p6 3d1 Ex) Strontium 38 e’s 2p6 4s2 1s2 2s2 3s2 3p6 3d10 4p6 5s2

  15. Afbau sequence from Periodic Table The periodic table can be used to determine theafbauorder instead As you increase the # electrons, the block structure indicates the sequence of subshells We can appreaciate that the origin of the periodic table is the electron configurations of the elements p block d block s block f block

  16. A more detailed look at the block structure 1 2 2 3 3 3 4 4 5 4 5 6 5 6 7 6 4 5

  17. 2p6 4s2 4p6 5s2 1s2 2s2 3s2 3p6 3d10 2p6 4s2 1s2 2s2 3s2 3p6 3d10 4p6 5s2 [Kr] Electron Configurations Electron configurations for the larger elements are lengthy to write out. noble gas notation - the symbol for a noble gas is used as an abbreviation for its electrons. The core electronsare represented by the noble gas followed by configuration of the valence electrons. Ex) Ne has an electron configuration of 1s22s22p6. For Na, we can write either 1s22s22p63s1 or [Ne]3s1 Ex)Sr 38 Kr 36 Core e’s Valence e’s

  18. Exercises Identify the elements with the following electron configurations. a) 1s22s22p3 c) [Ne]3s23p3 b) 1s22s22p63s23p64s23d7 d) [Kr]5s24d5 P N Tc Co How many core and valence electrons do these atoms have? a) ____core, ____valence c) ____core, ____valence b) ____core, ____valence d) ____core, ____valence 2 5 10 5 18 9 36 7

  19. Exceptionsto the aufbau order Exceptions to Afbau order result of: Full shell stability Half Shell stability Stability of higher spin state Cr Cu

  20. 2p6 4s2 4p1 1s2 2s2 3s2 3p6 3d10 Valence Revisited Electron configurations for fourth row from Gallium and beyond. Ex) Ga At. No. = 31 Ex) Ar At. No. = 18 1s22s22p63s23p6. Then for Ga we should write: [Ar]4s23d104p1 What isthevalencefor Ga? It has 3d10 electron that belong to the 3 shell not the 4 shell, hence it is strictly speaking not part of the valence if it is complete and should be considered as part of the core. Therefore: [Ar]3d10 is the core and 4s24p1 is the valence How about Thallium 81?

  21. Electron configurations of ions Electron configurations of ions can be determined from that of the neutral atom, i.e. electron configurations predictions Oxide forming from oxygen: O =1s22s22p4 O2- = 1s22s22p6 Ne = 1s22s22p6 Same electron configuration as neon Magnesium cation from magnesium: Mg2+ = 1s22s22p6 Ne = 1s22s22p6 Mg =1s22s22p63s2 Same electron configuration as neon This rationalizes the kinds of stable ions that are formed for certain elements

  22. Electron configurations of ions Thus, cation electron configuration is obtained by removing electrons in the reverse Aufbau sequence Anion electron configurations are obtained by adding electrons in the usual Aufbau sequence Ions try to achieve: (1) the closest noble gas configuration (2) a pseudo noble gas configuration (closed d or f subshell) (3) a noble gas configuration for everything exceptd or f electrons Cations always have their electron configurations in the sequence of the H. E.C of Element Core Valence Nearest stable Core • Li+ 2s1 = 1 e- 1s22s1 2. Br- [Ar]3d10 4s24p5 = 7 e- [Ar]4s23d104p6=[Kr] [Ar]4s23d104p5

  23. Electron configurations of ions E.C of Element Valence Nearest stable Core • C4+ 2s22p2 1s22s22p2 2. P3- 3p3 [Ne]3s23p3 3. Ga3+ 4s24p1 [Ar]3d104s24p1 4. Sn2+ [Kr]5s24d105p2 5p2 5. Sn4+ 5s25p2 [Kr]5s24d105p2

  24. Exercise Which of the following ions are likely to form? For those which are not what ion would you expect to form from that element? • O2- c) Cl+ e) Pb4+ • b) Mg6- d) Ca+ f) Ga3+ a) 8 O = 1s22s22p4 10 O2- =1s22s22p6 = Ne b) 12 Mg = 1s22s22p63s2 18 Mg6- = 1s22s22p63s2 3p6 = Ar c) 17 Cl = 1s22s22p6 3s23p5 16 Cl1+ = 1s22s22p63s23p4 d) Ca = 1s22s22p6 3s23p6 4s2 20 19 Ca1+ = 1s22s22p6 3s23p6 4s1 e) Pb = [Xe]6s24f145d106p2 Pb4+ = [Xe]4f145d10 82 78 f ) 28 31 Ga = [Ar]4s23d104p1 Ga3+ = [Ar]3d10

  25. Order of Energy Levels in Ions “Aufbau” energy levels: s below d “Aufbau” energy levels In Cations Energy levels in Anions Electrons less strongly bound as e-n interaction are decreased Electrons more strongly bound as e-n interaction are increased

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