Carbon

No other element bonds to itself so readily and in so many geometries, which is why there are tens of millions of known carbon compounds and only a few hundred thousand of everything else combined. The same atom gives diamond, the hardest natural substance, and graphite, one of the softest, differing only in whether every atom is bonded tetrahedrally or in stacked sheets — and the sheets, peeled off one at a time with sticky tape by Geim and Novoselov in 2004, are graphene. Carbon-12 was set at exactly 12 u in 1961 to end a decades-long split between chemists' and physicists' mass scales, and the mole was defined against it until 2019. The ±0.002 in its atomic weight is real natural variation: photosynthesis prefers the lighter isotope, which is how laboratories catch cane sugar smuggled into honey.