Atomic Mass Does Not Seem to Add Up

The masses of the electron, the proton, and the neutron have been determined, and the values are listed below.

me= 9.1093897 x 10-31 kg
mp = 1.6726231 x 10-27 kg
mn = 1.6749286 x 10-27 kg
1 amu = 1.6605402 x 10-27 kg
NA = 6.0221367 x 1023 mol-1
c = 299,792,457 m/s

Since 12C is defined as having a mass of exactly 12 amu, 1 mole of 12C has a mass of exactly 12 g. There are 6.0221367 x 1023 carbon atoms in a mol of 12C atoms; so

Many people notice that the mass of 12C is not equal to the sum of the masses of 6 electrons, 6 protons, and 6 neutrons.

Why is a 12C atom slightly lighter that the sum of all its parts?

Energy is released as a carbon atom forms from subatomic particles. That is, some of the mass of the subatomic particles is converted to energy and released when a carbon atom forms.