Traumatic, electrons are subatomic particles. They may not weigh much, but if youre counting particles, they are in there and ruin your definition.
The definition uses the number of neutrons+protons in an atom of a specific isotope. There is a caveat even to this though, as a fellow on reddit blew my mind with the explanation for the actual atomic weight of 1
The value of H = 1 is just a rough measure. Nucleons change mass a bit when they combine in a nucleus, the amount lost is called the mass defect. The mass of a proton is 1.672623E-27 kg, mass of a neutron is 1.674929E-27. So two protons and two neutrons should be 6.695104E-27 kg, however the actual mass of a helium nucleus is 6.644657230E−27 kg, which is lower than expected by 0.050447E-27 kg, which is off by about 1%. You can convert this mass defect to an energy using E = mc2 to get the nuclear binding energy.
So while the mass of one C-12 is almost 12 times the mass of hydrogen, it isn't exactly 12 times. 1 mole of hydrogen atoms is actually 1.007825 grams (about 1% off)., due to the mass defect lowering the mass of carbon.
Deuterium only accounts for 0.0115% abundance, and tritium is even more negligible, so that extra weight isn't much due to isotopic abundance. I never caught this before.