1. The problem statement, all variables and given/known data "You might wonder how six protons and six neutrons, each having a mass larger than 1 u, can be combined with six electrons to form a carbon-12 atom having a mass of exactly 12 u. The bound system of carbon-12 has a lower rest energy than that of six separate protons and six separate neutrons. According to E=mc^2, this lower rest energy corresponds to a smaller mass for the bound system. The difference in mass accounts for the binding energy when the particles are combined to form the nucleus." 2. Relevant equations E=mc^2 1amu= 1.660539e-27 kg 3. The attempt at a solution I came across this passage in my textbook, and I'm having difficulty in seeing how the rest energy for carbon-12 can be lower than that of six separate protons or six separate neutrons. My understanding is that the more mass the greater the rest energy.