This document provides a tutorial on periodic trends in physical and chemical properties for elements in periods 2 and 3 of the periodic table. It discusses how ionization energy generally increases across a period as the nuclear charge increases, but may decrease between certain elements due to electrons entering new orbitals farther from the nucleus or due to increased electron-electron repulsion with multiple electrons in the same orbital. Specifically, it notes the ionization energy decreases from beryllium to boron and from nitrogen to oxygen in period 2, and from magnesium to aluminum and from phosphorus to sulfur in period 3 for these reasons.