Safety and Accuracy Notice
This is a reference about what the elements are. It is not, anywhere, a reference about what to do with them.
Facts about substances, never instructions
Every hazard statement on this site is written in the same register as a boiling point: a property of a material, reported. Nothing here describes how to obtain, handle, store, transport, prepare, combine, dispose of or experiment with any substance, and no page contains a demonstration.
That is a deliberate constraint on the writing rather than a gap someone means to fill later. A reference that mixes "cesium reacts violently with water" into the same paragraph as an implication about how one might observe it has changed what kind of document it is. The prose here stays on the first side of that line, and the historical stories it tells — a poisoning case, a banned industrial process, a decommissioned reactor — are told as history.
What a hazard flag covers, and what it does not
The flags on an element page classify the pure element as characterised in published safety references. That is a narrower claim than it can look like.
- A flag on an element is not a flag on its compounds. Sodium is violently water-reactive; sodium chloride is table salt. Chlorine is an acute respiratory hazard; the chloride ion in seawater is not.
- Concentration, physical form and route of exposure change the picture entirely, and a one-word classification carries none of them. Chromium in stainless steel and chromium in a hexavalent industrial compound are not comparable exposures.
- Radioactivity here means an element has no stable isotope. It is a statement about nuclear structure, not a measure of how hazardous a particular sample would be, which depends on the isotope, its activity and its chemistry.
For any real material in any real setting, the documents that actually govern the question are the supplier's safety data sheet for that product and the regulations that apply where you are. Neither is reproduced here.
Where the numbers have limits
The methodology page names every source and the date each was captured. Three limits follow from that and are worth stating plainly.
Snapshots age. IUPAC revises standard atomic weights most years, and a value read here is the value that was checked on the capture date rather than the value published this morning.
Properties above element 103 are largely predicted from position rather than measured, because in most cases only a handful of atoms have ever existed. Where a figure of that kind appears, the page says what kind of figure it is.
Calculators here are working tools for learning and for checking your own arithmetic. They round, they use conventional atomic weights where an element has an interval, and they are not validated for any purpose where a wrong answer has a consequence. Independent verification is on you.
What this site is not
It is not medical advice, occupational health guidance, a regulatory compliance document, an engineering specification or a laboratory authority. It is a well-sourced reference written by one person, and it is useful precisely as far as that goes.