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 ?

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 angler momentum values Recall, also that when electrons move they generate a magnetic field, B. v B This is analogous to electrical current moving through a loop

Electrons in orbit generate magnetic fields, which requires all materials to be magnetic. Is that so?Why?How is this possible? Imagine two electrons in the same orbit moving in opposite directions. B v v B Electron Spin and Magnetism The magnetic fields cancel !! Do electrons occur in pairs in orbitals!!! 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. Yes!

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

Electron Spin and Magnetism When an external magnetic field is applied the electron will either along or against the field. Being aligned with the field is more stable than against, therefore the up orientation is slightly favored 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 More stableLess stable The distribution of up to down depends on strength of the applied magnetic field. B UP (s=1/2)DOWN (s=-1/2) Magnetic field

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

Ferromagnetic Materials – Have a permanent magnetic field 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. – i.e Magnets When two electron on separate atoms are close, the field from one will cause the other to align with it since it is more stable Magnetic Materials

Pauli Exclusion Principle Fermions - particles have spin ½. electronsprotonsneutrons “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. Ex) 1s orbital n = 1, l = 0, m = 0 and s = 1/2 or s = -1/2 1s Orbital

Atoms with more than one electron The wavefunctions for multi electron atoms 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

B 5 C 6 N 7 O 8 F 9 Ne 10 1s2p2s Hunds rule Element # e’s Electrons added to each empty orbital in parallel When no new orbitals are available they are paired Maximize spin

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)

B 5 C 6 N 7 O 8 F 9 Ne 10 1s2p2s Electron Configuration Element # e’s 1s 2 2s 2 2p 1 1s 2 2s 2 2p 2 1s 2 2s 2 2p 3 1s 2 2s 2 2p 4 1s 2 2s 2 2p 5 1s 2 2s 2 2p 6

Aufbau order and Energy Levels The sequence of subshells in the electron configurations not exactly 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 in subshells are still degenerate, the subshells are no longer degenerate in each shell, and differ in energy as s < p < d, Some subshells can overlap the levels of a different shell; thus, for example, in neutral atoms 4s lies below 3d

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 1s 2 2s 2 2p 5 Ex) Scandium 21 e’s 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 1 Ex) Strontium 38 e’s 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2

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

A more detailed look at the block structure

The core electrons are represented by the noble gas followed by configuration of the valence electrons. Electron configurations for the larger elements are lengthy to write out. Ex) Ne has an electron configuration of 1s 2 2s 2 2p 6. For Na, we can write either 1s 2 2s 2 2p 6 3s 1 or [Ne]3s 1 Electron Configurations Ex) Sr 38 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 Kr 36 [Kr] 5s 2 Core e’s Valence e’s noble gas notation - the symbol for a noble gas is used as an abbreviation for its electrons.

How many core and valence electrons do these atoms have? a) ____core, ____valence c) ____core, ____valence b) ____core, ____valence d) ____core, ____valence Identify the elements with the following electron configurations. a) 1s 2 2s 2 2p 3 c) [Ne]3s 2 3p 3 b) 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 7 d) [Kr]5s 2 4d 5 Exercises N Co P Tc

Exceptions to the aufbau order Exception to Afbau order result of:Full shell stability Half Shell stability Stability of higher spin state

Valence Revisited Electron configurations for fourth row Gallium and beyond. Ex) Ar has an electron configuration of 1s 2 2s 2 2p 6 2s 2 2p 6. Then for Ga we should write: [Ar]3d104s 2 3d 10 4p 1 Ex) Ga31 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 1 It 3d 10 electron 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. What is the valence for Ga? Therefore: [Ar]3d 10 is the core and 4s 2 4p 1 is the valence How about Thallium?

Electron configurations of ions Electron configurations of ions can be determined from that of the neutral atom, i.e. electron configurations predict ions Oxide forming from oxygen : Same electron configuration as neon This rationalizes the kinds of stable ions that are formed for certain elements O = 1s 2 2s 2 2p 4 O 2- = 1s 2 2s 2 2p 6 Ne = 1s 2 2s 2 2p 6 Mg = 1s 2 2s 2 2p 6 3s 2 Mg 2+ = 1s 2 2s 2 2p 6 Magnesium cation from magnesium: Ne = 1s 2 2s 2 2p 6 Same electron configuration as neon

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 except d or f electrons Cations always have their electron configurations in the sequence of the H. 1.Li + Nearest stable CoreValence 2s 1 = 1 e - 2.Br - [Ar]4s 2 3d 10 4p 5 5s 2 5p 5 = 7 e - [Ar]5s 2 4d 10 5p 6 =[Kr] Core [Ar]4d 10 E.C of Element

Electron configurations of ions 1.C P Ga Sn Sn 4+ Nearest stable Core 1s 2 2s 2 2p 2 Valence 2s 2 2p 2 3p 3 4s 2 4p 1 E.C of Element 5p 2 5s 2 5p 2

a)O 2- c) Cl + e) Pb 4+ b) Mg 6- d) Ca + f) Ga 3+ Which of the following ions are likely to form? For those which are not what ion would you expect to form from that element? Exercise O 2- =1s 2 2s 2 2p 6 O = 1s 2 2s 2 2p 4 = Ne Mg = 1s 2 2s 2 2p 6 3s Mg 6- = 1s 2 2s 2 2p 6 3s 2 3p 6 = Ar Cl = 1s 2 2s 2 2p 6 3s 2 3p 5 17Cl 1+ = 1s 2 2s 2 2p 6 3s 2 3p 4 20 Ca = 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 Ca 1+ = 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 Pb = [Xe]6s 2 4f 14 5d 10 6p 2 Pb 4+ = [Xe]4f 14 5d Ga = [Ar]4s 2 3d 10 4p 1 31Ga 3+ = [Ar]3d 10 a) b) c) d) e) f )

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