To expand on that, for physicists Carbon-12 is exactly 12 daltons also denoted 12 Da or 12 u. All other isotopes deviate slightly from a whole number of daltons

To expand on that, for physicists Carbon-12 is exactly 12 daltons also denoted 12 Da or 12 u. All other isotopes deviate slightly from a whole number of daltons. If I recall correctly the number of nucleons minus the daltons continues to decrease until you get to iron, which has the lowest nuclear binding energy, after which it continues to increase again (although there's some variation between isotopes so this is more of a general trend).