It is a unit of mass used to express atomic and molecular weights. It is equal to 1/12 of the mass of an atom of carbon-12.
An atom of carbon-12 has 6 protons and 6 neutrons in its nucleus.
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Mass unit equal to exactly one-twelfththe mass of one atom of carbon-12 is called one atomic mass unit. It is written as u.the isotope of carbon with six protons and six neutrons in its nucleus.