Understanding the periodic table is fundamental to chemistry. Learn about atomic structure, atomic number and atomic mass, electron configurations, and how the elements interact to form compounds — then put it into practice on the interactive periodic table.
Chemical elements are pure substances that cannot be broken down into simpler substances by chemical means. Each element is made up of atoms that have the same number of protons in their nucleus, known as the atomic number.
The periodic table organizes elements by increasing atomic number. Elements in the same column (group) have similar properties, while elements in the same row (period) have the same number of electron shells.
Elements in the same group have similar chemical properties because they have the same number of electrons in their outermost shell.
Elements in the same period have the same number of electron shells, making their atoms similar in size.
Every element belongs to one of these categories, each with its own color on the periodic table and its own set of shared properties:
Group 2 metals. Reactive and shiny, but less so than alkali metals.
Examples: Magnesium, Calcium, Barium
Softer metals to the right of the transition block, with lower melting points.
Examples: Aluminum, Tin, Lead
Group 18 gases with full outer shells, making them stable and chemically inert.
Examples: Helium, Neon, Argon
The first row of rare-earth metals (57–71), used in magnets and electronics.
Examples: Cerium, Neodymium, Europium
Radioactive rare-earth metals (89–103), including uranium and plutonium.
Examples: Thorium, Uranium, Plutonium
The number of protons in an atom's nucleus. This defines what element it is.
The average mass of all isotopes of an element, measured in atomic mass units.
Describes how electrons are distributed in an atom's orbitals.
Elements combine by sharing or transferring electrons to form compounds.
Each tile on the periodic table packs four key facts about an element. Once you can read one tile, you can read all 118.
The table’s real power is that properties change in predictable patterns. These periodic trends let you estimate how an element behaves just from where it sits:
Decreases across a period (left to right) and increases down a group.
Energy needed to remove an electron. Increases across a period, decreases down a group.
An atom’s pull on shared electrons. Highest at the top-right (fluorine), lowest at the bottom-left.
Decreases across a period and increases down a group — metals sit on the left, nonmetals on the right.
In 1869, Russian chemist Dmitri Mendeleev arranged the known elements by atomic mass and noticed their properties repeated at regular intervals. His Mendeleev table was so reliable that he left gaps for elements not yet discovered — and correctly predicted their properties.
The modern periodic table refines his idea by ordering elements by atomic number rather than mass, which resolved the few cases where Mendeleev’s order seemed out of place. Today it holds all 118 confirmed elements and remains chemistry’s most important map.