How to use the periodic table
- Click any element, or type a name, symbol or atomic number in the search box: Fe, iron and 26 all find iron.
- Read the details card: atomic number, standard atomic weight with its uncertainty, group, period, block and category. Copy the symbol or weight with one click.
- Press a category such as Noble gas to highlight that family. On a phone the table becomes a list you can search the same way.
How the table is arranged
Each row is a period: elements in the same period have the same number of electron shells. Each column is a group: elements in a group have the same number of outer electrons, so they behave in similar ways. Sodium and potassium in group 1 both react strongly with water, and the noble gases in group 18 barely react at all.
The blocks follow which orbitals are being filled. Groups 1 and 2 (plus helium) are the s-block, groups 13 to 18 the p-block, groups 3 to 12 the d-block (the transition metals), and the lanthanides and actinides the f-block.
Atomic weights and isotopes
Most elements are a mix of isotopes, so their atomic weight is an average. For some, such as lithium, boron and sulfur, that mix varies from place to place in nature, so IUPAC gives a wider uncertainty: lithium is 6.94 ± 0.06. For calculations like molar mass, the value in this table is the right one to use; the molar mass calculator uses the same data.
Categories
The colour groups (alkali metals, halogens, noble gases and so on) are the usual way of sorting elements by behaviour, but they are not all official IUPAC terms and different books draw some borders differently. Here the metalloids are boron, silicon, germanium, arsenic, antimony and tellurium, and elements 109 to 118 are marked as having unknown properties because only a few atoms of each have ever been made.