Interactive Periodic Table

How it works

The table is ordered by atomic number, which is the number of protons. Elements in the same column, called a group, have the same number of outer electrons, so they behave alike: group 1, the alkali metals, are soft and reactive; group 17, the halogens, are reactive nonmetals; group 18, the noble gases, barely react at all. Across a row, called a period, the atoms get heavier and the outer shell fills up.

Weights in square brackets, like [98] for technetium, are not standard atomic weights. Those elements have no stable form, so the bracket shows the mass number of the longest-lived isotope instead. The two rows at the bottom hold the lanthanides and actinides, the f-block, which sit in the gaps of periods 6 and 7 so the table stays a readable shape.

Colour the table by electronegativity and a pattern jumps out: it rises across a period and falls down a group, so fluorine, at the top right, holds its electrons tightest at 3.98, while caesium and francium at the bottom left barely hold them at all. That is why fluorine is so reactive and the alkali metals give up an electron so easily. The electron configuration tells the same story in another way: the outer shell fills across a period, and a full outer shell, like neon or argon, is content and unreactive. Worked example: iron, Fe, is element 26 with a mass of 55.845, the configuration [Ar] 3d⁶ 4s², and the common ions Fe²⁺ and Fe³⁺ — the two charges it takes in rust and in haemoglobin. Try searching “na” to find sodium first, typing 17 to open chlorine, or switching Colour by to year discovered to see which elements chemists knew in ancient times and which were only made in the last hundred years.