What nuclide is used as the standard in the relative scale for atomic masses?

The correct answer is Carbon-12.

Explanation:

The relative isotopic mass is the relative mass of a given isotope, scaled with carbon-12 as exactly 12 u. There are no other nuclides other than carbon-12 that have exactly an integer number of masses on this scale. This is due to two factors:

1- The different mass of neutrons and protons acting to change the total mass in nuclides with proton/neutron ratios other than the 1:1 ratio of carbon-12.

2- An exact number will not be found if there is a different mass gain/loss than the nuclear bond energy relative to the mean nuclear bond energy of carbon-12.

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