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Ch. 5.2 Electron Configuration in Atoms Electron Configurations • Determined by three rules: the aufbau principle, the Pauli exclusion principle, and Hund’s rule. Aufbau Principle • Electrons occupy orbitals of lowest energy first. • Use the aufbau diagram (figure 5.7) to see the energy levels of various atomic orbitals. • The filling of atomic orbitals does not follow a simple pattern beyond the second energy level. Pauli Exclusion Principle • An atomic orbital may describe at most two electrons. • Electron spin is a quantum mechanical property; may be thought of as clockwise or counterclockwise. • Two electrons must have opposite spins to occupy the same orbital. • Spin direction can be indicated with either an up or down arrow. Hund’s Rule • When using the aufbau diagram, one electron enters each orbital until all the orbitals contain one electron with the same spin direction. • The number of electrons with the same spin direction is as large as possible. Shorthand for Electron Configuration • Write the energy level and the symbol for every sublevel occupied by an electron. • Indicate the number of electrons occupying that sublevel with a superscript. • Example: Hydrogen is 1s1. • Helium is 1s2. • Oxygen is 1s22s2p4. • The sum of the superscripts indicates the number of electrons in the atom. • When electron configurations are written, sublevels with the same energy level are generally written together (may conflict with the order of the aufbau diagram). Exceptional Electron Configurations • Some elements have electron configurations that are exceptions to the aufbau principle. Because… • Half-filled sublevels are not as stable as filled sublevels, but are more stable than other configurations, due to subtle electronic interactions in orbitals with similar energies. • • • • • Example: Chromium—using aufbau… Cr 1s22s22p63s23p63d44s2 wrong Cr 1s22s22p63s23p63d54s1 right! Its more important to understand the rules for determining electron configuration when the aufbau rule applies!