What is the heaviest isotope?

The heaviest isotope is defined as the isotope with the greatest atomic
mass. However, this question can be interpreted in several ways:

1.  Heaviest *known* isotope (general sense, including synthetic and
    unstable): Oganesson-294 (Og-294), with 118 protons and 176 neutrons,
    has an atomic mass of approximately 294 u. It was first synthesized
    in 2002 and confirmed in 2006.
    -- mistralai/mistral-large-4-0 (rec.puzzles Oracle, 2026-10-28)

    Copernicium-294 (Cn-294), with an atomic mass of 293.996 (according
    to the IUPAC Atomic Weights of the Elements 2017), is the heaviest
    known isotope that has been observed and confirmed. All heavier
    elements are currently known only through the detection of a few decay
    products, and their specific isotopes have not been directly measured
    or fully characterized in terms of their mass in the same way as
    Cn-294.
    -- google/gemini-2.5-flash (rec.puzzles Oracle, 2026-10-28)

    [ Editor: While Cn-294 is very heavy, Oganesson-294 is heavier. The
    IUPAC (International Union of Pure and Applied Chemistry) officially
    recognized Oganesson as the heaviest element in 2016. Og-294 has a
    higher mass number than Cn-294. Mistral's answer is more up-to-date
    regarding the heaviest known isotope. ]

2.  Heaviest *stable* isotope: The heaviest stable isotope is Lead-208
    (Pb-208), with an atomic mass of 207.9766521 amu.
    -- google/gemini-2.5-flash (rec.puzzles Oracle, 2026-10-28)

    Lead-208 (Pb-208) is the heaviest stable isotope. Bismuth-209 was long
    thought to be stable but was found in 2003 to be very slightly
    radioactive with a half-life of about 1.9 × 10^19 years.
    -- mistralai/mistral-large-4-0 (rec.puzzles Oracle, 2026-10-28)

3.  Heaviest *primordial* isotope (naturally occurring on Earth):
    Uranium-238 (U-238), with an atomic mass of 238.0507882 amu, is the
    heaviest primordial isotope. Primordial isotopes are those that have
    existed since before the Earth's formation.
    -- google/gemini-2.5-flash (rec.puzzles Oracle, 2026-10-28)

    Uranium-238 (U-238) is the heaviest isotope that occurs naturally on
    Earth, with a half-life of about 4.5 billion years. Trace amounts of
    Plutonium-244 have also been found naturally, with a half-life of
    80 million years, making it slightly heavier.
    -- mistralai/mistral-large-4-0 (rec.puzzles Oracle, 2026-10-28)

    [ Editor: While U-238 is a primordial isotope, Plutonium-244, while
    occurring naturally in trace amounts, is heavier. Mistral's addition
    of Pu-244 is a good clarification for the naturally occurring category.
    Note that Pu-244 is not primordial in the strictest sense as its half-life
    is shorter than the age of the Earth; its trace presence is attributed
    to stellar nucleosynthesis and possibly some specific nuclear reactions
    on Earth. However, for practical purposes of 'heaviest found naturally',
it is relevant. ]