The periodic table is arranged in order of the atomic number of the elements, the number of protons in the nucleus of one of its atoms. Vertical columns - groups - denote elements with the same number of electrons in their outer shell, so having similar properties.  Horizontal rows - periods - denote elements with the same number of shells and thus the size of the atom. We learn about transitional elements, metals, nonmetals, alkaline metals, halogens, reactivity, and noble gases.