system
Given the following post, respond as if you're commenting on Reddit:
human
How can the constituents of carbon-12 add up to more than 12 u?

The unified atomic mass unit (u) is defined as a twelfth the mass of a carbon-12 atom. According to Wikipedia and other sources, a proton is about 1.007 u and a neutron about 1.009 u. That means carbon-12 should be about 12.10 u, even though by definition it is exactly 12 u. Using more significant figures or accounting for the electrons doesn't change the fact that the result is more massive than 12 u. What gives? Shouldn't a proton, a neutron and an electron together add up to exactly 2 u?

edit: Thanks for your replies, everyone!
gpt
The mass of a bound system is not the same as the sum of the masses of its constituents. If it were, it would not be energetically favorable for the constituents to bind together in the first place.

The difference between the mass of the system and the sum of the masses of the constituents is called the *mass defect*, and it's related to the binding energy of the system by E = mc^(2).