The total mass of six protons, six neutrons and six electrons is 12.0993 amu, slightly more than a carbon-12 atom's 12.00 amu, which shows that:

1. The unified atomic mass unit is defined so that carbon-12 weighs exactly 12.00 amu. 2. Adding up its separated particles gives 12.0993 amu instead. 3. The sum of an atom's subatomic particles does not equal the atom's actual mass. 4. The bound nucleus is therefore lighter than its separated components. 5. The definition is not at fault, and electrons do have mass, so the other options fail. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 2 'Atoms, Molecules, and Ions', sections 2.1-2.6_