2.1: Historical Development of Atomic Theory 2.2: The Periodic Table 2.3: Discovery of Subatomic Particles and the Bohr Atom 2.4: The Schrödinger equation, particle in a box, and atomic wavefunctions 2.4.1: Particle in a Box 2.5: Quantum Numbers and Atomic Wave Functions 2.6: 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. 2.7: Shielding 2.8: 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. 2.8.1: Covalent and Ionic Radii 2.9: Ionization energy 2.10: Electron Affinity 2.11: Lanthanide Contraction The Lanthanide Contraction describes the atomic radius trend that the Lanthanide series exhibit. Another important feature of the The Lanthanide Contraction refers to the fact that the 5s and 5p orbitals penetrate the 4f sub-shell so the 4f orbital is not shielded from the increasing nuclear change, which causes the atomic radius of the atom to decrease. This decrease in size continues throughout the series.