Neutrons and protons have a mass equal to _____________.

• A. 1 amu
• B. 2 amu
• C. 1.67 X 10^-27 kg
• D. 0.5 amu

Answer: (A)

Masses of protons and neutrons are essentially the same and set the scale for atomic masses. An atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom, so carbon-12 has exactly 12 amu. Protons and neutrons have masses close to 1 amu each (about 1.0073 amu for a proton and 1.0087 amu for a neutron), making them the reference for the nucleon mass. In kilograms, one amu is about 1.66 × 10^-27 kg, so a nucleon weighs roughly 1.66 × 10^-27 kg. Chemistry problems express atomic-scale masses in amu, so stating their mass as about 1 amu is the conventional and most appropriate answer. The exact kilogram value is correct but is not the unit typically used in this context, and 0.5 amu would be too small while 2 amu would be too large for a single nucleon.