The periodic table is a tabular arrangement of all known chemical elements, organized by increasing atomic number. It reveals periodic trends in chemical properties that recur across the table. Dmitri Mendeleev is credited with creating the modern version of this table in 1869. He arranged the elements by atomic mass and, remarkably, left gaps for elements that had not yet been discovered, predicting their properties with striking accuracy.
The table is divided into horizontal rows called periods and vertical columns called groups, or families. Periods indicate the energy levels of an atom's electrons, while groups contain elements with similar chemical behavior because they share the same number of valence electrons. Elements within the same group tend to react in comparable ways and form similar compounds.
Across the periodic table, elements are classified into three broad categories based on their properties. Metals, found on the left side and in the center of the table, are typically shiny, malleable, and good conductors of heat and electricity. Nonmetals, located on the right side, tend to be dull, brittle, and poor conductors. Along a staircase-shaped boundary between these two regions sit the metalloids, which exhibit properties intermediate between metals and nonmetals, making them valuable as semiconductors in electronic devices.