All Chemistry is a reference for chemistry: the periodic table with the measured properties of all 118 elements, a page for every element, and tools that work in the browser. It has been open to readers since 2013.
Where to begin
The periodic table — all 118 elements, recoloured by category, by state, by block or by electronegativity, and searchable by symbol or by name.
A page for every element — atomic mass, electron configuration, oxidation states, ionisation energy, melting and boiling points, density and the year of discovery, with the properties of the atom kept apart from those of the simple substance: oxygen, carbon, iron.
The structural formula editor — draw a molecule or write out a reaction, with nothing to install.
News — short notes on results worth the attention.
One thing worth knowing first
Oxygen the element has an electron configuration. Oxygen the substance boils at 90 K — and ozone, built from exactly the same atoms, boils at 161 K. A property of the atom holds wherever that atom is; a property of the substance describes a piece of material under stated conditions. Every element page keeps the two apart, because running them together is how "oxygen is a gas" comes to be taken for a fact about the element.