Deuteron

SPIN: 1
SYMBOL: d
CHARGE: +1 e
MASS: 1875.612945 ± 5.8 × 10⁻⁷ MeV/c²
CODATA SOURCE: CODATA 2022

Nucleus of deuterium — one proton and one neutron bound into the simplest composite nucleus.

Harold Urey discovered deuterium in 1931 and won the Nobel Prize in Chemistry three years later.
Its binding energy is just 2.22 MeV — so weakly bound that no two-proton or two-neutron nucleus exists at all.
Deuterium’s survival out of Big Bang nucleosynthesis makes it a sensitive probe of the early universe.