How to use the interactive periodic table
- Click or tap any element to see its atomic number, atomic mass, category, group, period, state at room temperature and electron configuration.
- Search by name, symbol or atomic number, or type a category such as "halogen" or "noble" to highlight a family.
- Switch the colours to show categories, states of matter at 20 °C, or atomic mass from light to heavy. On a phone, scroll the table sideways.
How the periodic table is organised
The 118 known elements are arranged in order of atomic number, the number of protons in the nucleus. Rows are periods: each new period starts a new electron shell. Columns are groups: elements in the same group have the same number of outer electrons, so they behave in similar ways. That is why the alkali metals in group 1 all react vigorously with water, the halogens in group 17 form salts with metals, and the noble gases in group 18 hardly react at all. The lanthanides and actinides are usually shown in two separate rows underneath to keep the table a sensible width.
Atomic masses are the IUPAC standard atomic weights, rounded, in unified atomic mass units (u), which are also grams per mole. For elements with no stable isotopes, the mass of the longest-lived isotope is shown in brackets. Use them with our Molar Mass Calculator to work out the mass of compounds. Dmitri Mendeleev published the first widely recognised periodic table in 1869, leaving gaps for elements that were discovered later, exactly as he predicted.
How many elements are there?
118 have been discovered and named, from hydrogen (1) to oganesson (118). Elements beyond 94 are made artificially.
What is the difference between a group and a period?
A group is a column of elements with similar chemistry. A period is a row; elements in a period have the same number of electron shells.
Which elements are liquid at room temperature?
Only mercury and bromine. Gallium and caesium melt just above room temperature.