system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the atomic mass of say, Chlorine-35 known to be about 34.96885 amu, if both one proton and one neutron weigh more than 1 amu?

It doesn't make any sense to me as a math student. I am aware that the atomic mass unit is defined by one twelfth the mass of one atom of Carbon-12, and also that a neutron weighs slightly more than a proton, but since Carbon-12 weighs 12 amu, and it has 6 neutrons and 6 protons,  protons should weigh slightly less than 1 amu, and neutrons should weigh slightly more than 1 amu, should they not?
gpt
The reason is that a stable nucleus has less rest energy than its separated constituents (in relativity, the rest energy of a system *is* its mass). You have to spend energy to separate the protons and neutrons inside a nucleus, and that energy is what makes for the difference in mass.