2.1: Historical Development of Atomic Theory 2.1.1: The Periodic Table 2.1.2: Discovery of Subatomic Particles and the Bohr Atom 2.2: The Schrödinger Equation and Atomic Wavefunctions 2.3: Aufbau Principle The Aufbau Principle (also called the building-up principle or the Aufbau rule) states that, in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy level before occupying higher-energy levels. 2.3.1: Shielding 2.3.2: The Aufbau Process 2.3.3: Electron Configuration of Transition Metals 2.4: Periodic Properties of Atoms General periodic trends are specific patterns that are present within the periodic table; these are patterns in properties like electronegativity, ionization energy, electron affinity, atomic radius, melting point, and metallic character. General periodic trends provide chemists with an invaluable tool to quickly predict an element's properties. These trends exist because of the similar atomic structure of the elements within their respective group families or periods. 2.4.1: Ionization energy 2.4.2: Electron Affinity 2.4.3: Covalent and Ionic Radii 2.5: Problems