Atomic mass is calculated from protons and neutrons alone because electrons:

1. Protons and neutrons are roughly two thousand times heavier than electrons. 2. The number of neutrons in an atom contributes significantly to its mass, but not to its charge. 3. When considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom's mass based on the number of protons and neutrons alone. 4. Electrons certainly do carry charge, which is why the second option is wrong. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 2 'The Chemical Foundation of Life', sections 2.1-2.3_