Question

Why atomic mass of carbon is 12 that of the helium is 4

Answer 1

This is because of the sum of the no of protons and neutrons

Answer 2

For example, one atom of helium-4 has a mass of 4.0026 amu. ... The carbon-12 atom has six protons and six neutrons in its nucleus for a mass number of 12. Since the nucleus accounts for nearly all of the mass of the atom, a single proton or single neutron has a mass of approximately 1 amu.