How does the organization of the Periodic Table help predict the properties of elements?

The periodic table arranges elements by increasing atomic number and groups them so that elements with similar electron configurations appear in the same columns, called groups. Because electrons in the outer shell determine chemical behavior, elements in a group share similar valence electron patterns, which means they react in comparable ways—alkali metals, for example, all lose one electron to form +1 ions. Moving down a group, the extra inner shells make atoms larger and less reactive, so the table’s vertical layout shows how properties change gradually. The rows, or periods, show how adding a new energy level changes atomic size and ionization energy, so you can anticipate whether an element will be a metal or a nonmetal. For instance, sodium (Na) and potassium (K) are both in group 1; they both readily lose one electron and form +1 ions, which is why they are both soft, highly reactive metals.