Electron Configuration If e- prefer to be near the nucleus, then they will fill orbitals from nucleus out Remember Aufbau? Fill orbitals in order of increasing energy Low n to high n, s before p before d before f Must also consider Pauli Exclusion Principle and Hund’s Rule

Can you explain why orbital energy increases with increasing n? Hint: Coulomb’s Law and 2 What about s < p < d < f? Must consider shielding and penetration

Do all e- feel same pull/attraction from/to nucleus? Why or why not? Besides distance, must also consider shielding or screening

Zeff Core electrons shield or screen valence electrons from the charge of the nucleus Nuclear charge is not experienced the same by all electrons. Outer electrons experience it less, so Z is reduced. The new Z is called Zeff Zeff = Z – S where Z is actual nuclear charge (#p in nucleus) and S is shielding constant (# core e-)

How many protons and electrons? Can you write the e- configuration? Consider Li. How many protons and electrons? Can you write the e- configuration? Do the electrons in ring 1 feel same pull as the electron in ring 2? What charge do the ring 1 e’s feel? What charge does the ring 2 e- fell? What if the e- in the second ring could penetrate the first ring? Would the charge it feels change?

Penetration Space in the square: an electron from 2s penetrating ring 1 Space in the oval: an electron from 2p penetrating ring 1 There is less penetration for 2p so it becomes higher in energy than 2s because more shielding, less Coulombic attraction This explains Aufbau’s Building Up Principle and electron configurations

In 1905, Einstein explained the photoelectric effect If you shine light on a clean metal surface, e- can be emitted (as color or light) To eject an e-, the light hitting the sample must have a minimum energy (threshold energy) If not at threshold energy, no light will be emitted from sample (no matter the intensity or amount of light) Einstein assumed that the radiant energy hitting the surface was quantized and has Ephoton = h

PES: Photoelectron Spectroscopy PES measures the amount of energy needed to eject electrons The energy gives a sense of the binding energies of electrons in a substance. Electron binding energy is a measure of the energy required to free electrons from their atomic orbits (ionization energy) as increase E, harder to remove, so probably closer to the nucleus for ex

PES: Photoelectron Spectroscopy The binding energies of the measured electrons are characteristic of the chemical structure and molecular bonding of the material One of the most sensitive and accurate techniques for measuring the energies and shapes of electronic states and molecular and atomic orbitals Provides direct evidence for the shell model

PES http://www.chem.arizona.edu/chemt/Flash/photoelectron.html

Give e- configuration and identify the element