earth materials
Three numbers describe an atom, and geologists use all three for different purposes. Confusing them is the commonest error in writing about geochemistry — and one of them is not a single number at all.
About: Silicon · Oxygen · Potassium-40 · Oxygen-18 · Carbon-13
An element is defined by one number: how many protons its nuclei contain. Silicon has 14, oxygen has 8, iron has 26. Change that count and you have a different element, whatever else the atom shares.[1,2,3]
Nothing else does the job. Not mass, not chemistry, not appearance — atomic number, and only atomic number.[1]
Atoms of one element can carry different numbers of neutrons. The total count of protons and neutrons is the mass number, and it identifies a nuclide within its element: carbon-12 and carbon-13 are both carbon, and behave chemically almost identically.[1,2]
Almost identically, but not quite — and the difference is the whole of stable-isotope geochemistry. Because the heavier isotope is slightly harder to move, processes that involve evaporation, crystallisation or biological uptake leave the two in slightly different proportions.[1,2]
Some nuclides are unstable. Potassium-40 makes up about one atom in eight and a half thousand of natural potassium, and that trace is simultaneously one of the three main sources of Earth’s internal heat and the basis of potassium–argon dating.[1,2]
The standard atomic weight is a property of the element as it is actually found — a weighted average over the isotopes present in normal materials. It is not the mass of any single atom, and it is not a universal constant.[1]
For elements whose isotopic composition varies in nature, the recommended value is an interval rather than a figure. Oxygen is given as [15.99903, 15.99977]; carbon, silicon, sulfur, hydrogen and lead are treated the same way. The interval is not uncertainty about the measurement — it is the range the quantity genuinely takes.[1,2,3,4]
For elements whose composition does not vary appreciably, a single value with a stated uncertainty is enough. Aluminium is 26.9815384(3); iron is 55.845(2).[1,2]
That natural isotopic abundance varies at all is what makes isotopes useful. Oxygen-18 relative to oxygen-16 shifts with temperature; carbon-13 relative to carbon-12 shifts when carbon passes through living things. If the ratios were fixed, they would record nothing.[1,2,3]
Often said
“Every element has a fixed atomic weight.”
What the evidence supports
For elements whose isotopic composition varies between materials, the recommended standard atomic weight is an interval, not a single number. Oxygen, carbon, silicon, sulfur, hydrogen and lead are all given as intervals by the body responsible for evaluating them.
Supporting records: standard-atomic-weight-defined · oxygen-atomic-weight · isotope-abundance-varies
Often said
“Atomic weight and atomic number are two ways of saying the same thing.”
What the evidence supports
Atomic number is a count of protons and defines the element. Atomic weight is a mass averaged over the isotopes actually present, and can be an interval. They are different quantities measured in different ways for different purposes.
Supporting records: silicon-atomic-number · standard-atomic-weight-defined
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-06.