In the structure of the Periodic Table, elements are primarily organized by their __________, which influences their chemical behavior and properties.

In the Periodic Table, elements are primarily organized by their atomic number, the number of protons in the nucleus. This number determines how many electrons orbit the nucleus, which in turn dictates how the element reacts with others. Because the atomic number increases from left to right and top to bottom, elements with similar numbers of valence electrons end up in the same column, showing similar chemical behavior. For example, sodium (atomic number 11) and potassium (atomic number 19) both have one valence electron, so they both readily form +1 ions. Thus, the atomic number is the key organizing principle that links structure to reactivity.