In this chemistry lesson on periodicity, students learn about groups and periods in the periodic table, focusing on the significance of group numbers which represent the number of outer shell electrons in an atom, thereby determining the chemical properties of elements within a group. The lesson also discusses periods, which are horizontal rows that indicate the number of electron shells; as one progresses across a period, elements transition from metallic to non-metallic, with an increase in the number of valence electrons. By the end of the lesson, students should be able to utilize these trends to predict properties of elements or compounds within the same group or area of the periodic table.