What is the periodic table?
The periodic table arranges all 118 known chemical elements in order of atomic number – the number of protons in the nucleus of each atom. Rows are called periods and columns are called groups. Elements in the same group have the same number of outer electrons, so they behave in similar ways: the alkali metals in group 1 all react with water, while the noble gases in group 18 hardly react at all.
Dmitri Mendeleev published the first widely accepted version in 1869. He left gaps for elements nobody had found yet and predicted their properties; gallium, scandium and germanium were discovered over the next 17 years and matched his predictions. Today the table is complete up to element 118, oganesson.
How to read the periodic table
Each box shows one element: its atomic number at the top, its chemical symbol in the middle, then its name and its atomic mass in grams per mole.
The colors sort the elements into families such as the alkali metals, transition metals, halogens and noble gases. Metals fill the left and center of the table, nonmetals sit at the upper right, and the metalloids form a staircase between them.
The two rows at the bottom are the lanthanides and actinides. They belong in periods 6 and 7 but are shown separately so the table does not become too wide.
Hover over or tap any element to see its electron configuration, electronegativity, melting and boiling points, density and discovery year, then open its page for the full story.