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The periodic table arranges elements left to right and top to bottom by increasing atomic number. Elements with similar properties are grouped in vertical columns called groups or families. The table has seven rows (periods) and eighteen columns (groups), with elements ordered so that those sharing chemical behavior fall in the same group.

Metals, grouped left and middle, conduct electricity and heat well and are shiny and malleable. Nonmetals, on the right side, are poor conductors and often dull. Metalloids, along the metal-nonmetal divide, exhibit mixed properties and include semiconductors like silicon that conduct better at higher temperatures.

Alkali metals in group 1 are much more reactive than alkaline earth metals in group 2. Alkali metals form compounds with oxygen in a 2:1 ratio, while alkaline earth metals form 1:1 compounds. These differences arise from their distinct electron configurations and tendencies to form cations and anions.

Halogens in group 17 are highly reactive nonmetallic elements that readily gain electrons to form anions. Noble gases in group 18 are nonreactive nonmetallic elements. This dramatic difference in reactivity reflects their opposite tendencies: halogens need one electron to complete their outer shell, while noble gases already have complete outer shells.

Mendeleev used his periodic table to predict undiscovered elements with properties similar to aluminum and silicon. The later discoveries of gallium in 1875 and germanium in 1886 confirmed his predictions, providing strong evidence that elements follow a periodic relationship based on atomic number rather than atomic mass.

Main-group elements occupy columns 1, 2, and 13-18 and include alkali metals, alkaline earth metals, and halogens. Transition metals fill columns 3-12. Inner transition metals, called lanthanides and actinides, occupy two rows at the table's bottom and include many radioactive elements.

Elements like technetium, promethium, polonium, and heavier elements have atomic masses in square brackets because they consist entirely of unstable radioactive isotopes. An average atomic weight cannot be reliably determined since their radioisotopes vary significantly in abundance or may not exist naturally.