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Modern Atomic Theory Quantum Numbers Quantum Numbers Quantum Numbers Rules The Aufbau principle states that energy levels must be filled from the lowest to the highest and you may not move on to the next level unless the previous level is full. Use the periodic table as a guide Rules Hund’s Rule says that when placing electrons in orbitals of equal energy, place one in each orbital before doubling up in order to arrive at the lowest energy configuration. The Pauli Exclusion Principle states that when electrons do share an orbital, they must be of different “spin.” Rules Exceptions to the rules There are a few exceptions to the rules listed above when filling electron configurations. A half-full “s” orbital and a “d” subshell with 5 or 10 is more stable than following the Aufbau Principle. Cr, Mo, W: s1 d5 and Cu, Ag, Au: s1 d10 Electron Configurations Orbital information is often described in a shorthand notation called an electron configuration 1s22s22p3 Electron Configurations Represents the Shows the number of principal energy level electrons in the orbital 1s2 Indicates the shape of the orbital Orbital Diagrams This information is also described using orbital diagrams. Arrows are added to an orbital diagram to show the distribution of electrons in the possible orbitals and the relative spin of each electron __ __ __ __ __ 1s 2s 2p Writing Electron Configurations Determine the number of electrons in the atom from its atomic number Add electrons to the sublevels in correct order of filling s = 2, p = 6, d = 10, f =14 To check your complete electron configuration, look to see whether the location of the last electron added corresponds to the element’s position on the periodic table Drawing Orbital Diagrams Draw a line for each orbital of each sublevel mentioned in the complete electron configuration. Draw one line for each s sublevel, three for each p sublevel, 5 for each d sublevel and 7 for each f sublevel. Label each sublevel For orbitals containing two electrons, draw one arrow up and one arrow down to indicate the electrons’ opposite spins.
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