What Is the Periodic Table?
A cheat sheet for the entire universe — every element that exists, organized in one brilliant chart.
Everything Is Made of Elements
Look around you. Your phone, the air, your own body — everything is made of elements, pure substances that can't be broken down into anything simpler by chemical means. Hydrogen, oxygen, carbon, iron, gold — these are all elements. Scientists have discovered 118 elements so far, and the periodic table is the chart that organizes all of them in a way that reveals patterns and relationships.
How It's Organized
Elements are arranged by atomic number — the number of protons in an atom's nucleus. Hydrogen has 1 proton (atomic number 1), helium has 2, lithium has 3, and so on up to oganesson with 118. The table is arranged in rows called periods (7 periods) and columns called groups (18 groups). Elements in the same group share similar chemical properties — they behave in similar ways.
Metals, Nonmetals, and Metalloids
Most elements are metals — shiny, good conductors of heat and electricity, and usually solid at room temperature. They occupy the left and center of the table. Nonmetals sit on the upper right — they're often gases or brittle solids, poor conductors, and include essentials like oxygen, nitrogen, and carbon. In between sit metalloids (like silicon), which have properties of both.
Famous Groups
Group 1 (alkali metals) like sodium and potassium are so reactive they explode on contact with water. Group 17 (halogens) like chlorine and fluorine are highly reactive nonmetals. Group 18 (noble gases) like helium, neon, and argon are the opposite — they almost never react with anything because their electron shells are already full. The noble gases' stability is the key to understanding why atoms bond: most atoms are trying to achieve a noble-gas-like electron configuration.
Why It's Called "Periodic"
The word "periodic" means "repeating at regular intervals." Russian chemist Dmitri Mendeleev noticed in 1869 that when elements are arranged by atomic weight, their properties repeat in a regular pattern — metals, then metalloids, then nonmetals, then noble gases, over and over. He even left gaps in his table for elements that hadn't been discovered yet and predicted their properties. When those elements were found years later, his predictions were astonishingly accurate.
When Mendeleev created his periodic table in 1869, only 63 elements were known. He boldly left gaps and predicted the properties of three undiscovered elements he called eka-aluminum, eka-boron, and eka-silicon. When gallium was discovered in 1875, its properties matched eka-aluminum almost perfectly — even the density Mendeleev predicted was off by less than 1%. This stunning prediction convinced the scientific world that the periodic table wasn't just a convenient chart but revealed a deep truth about how matter is organized.
Last reviewed: April 2026