Flexi Says: The periodic table is an arrangement of the elements in order of their atomic numbers so that elements with similar properties appear in the same vertical column or group. A period is a horizontal row of the periodic table. There are seven periods in the periodic table, with each one beginning at the far left. A new period begins when a new principal energy level begins filling with electrons. A group is a vertical column of the periodic table, based on the organization of the outer shell electrons. There are a total of 18 groups.