|CH301H - Principles
of Chemistry I: Honors
Fall 2011, Unique 51040
Lecture Summary, 13 October 2011
It is known from experiments that electrons behave as little bar
magnets. This is quantified in a fourth quantum number known as
magnetic spin, ms, which can take on the value +/- 1/2. Therefore there are 4 quantum numbers that describe each electron in an atom.
Pauli Exclusion Principle: The Pauli exclusion principle says that in an atom, no two electrons can have the same four quantum numbers. Each set of quantum numbers is distinct for every electron. Therefore, two electrons which share the quantum numbers and n, l, and m MUST have different values of ms. Exceptions to the Pauli exclusion principle ARE NOT ALLOWED.
Aufbau and Hund's Rule: All of this information allows us to fill electrons into multi electron atoms. This is done following the Aufbau (or "building up") principle, in which electrons are put into orbitals of increasing energy while obeying the Pauli exclusion principle. If we are putting electrons into p or d orbitals, we will have the choice of filling several degenerate states. Hund's rule states that when putting electrons into degenerate orbitals, put a single electron into each orbital before adding a second electron to any orbital. Furthermore, single electrons in degenerate orbitals should all have the same spin. Excepts to Hund's rule will result in higher energy configurations, not the ground state (but ARE allowed if they do not violate Pauli).