Why is the amu of chlorine-35 less than 35? [duplicate]








**This question already has an answer here**:
 
 




[Why are the masses of atoms less than the sum of their subatomic particles?](/questions/27446/why-are-the-masses-of-atoms-less-than-the-sum-of-their-subatomic-particles)

 (1 answer)
 

Closed 3 years ago.



My book says that a proton weighs 1.0073u, a neutron weighs 1.0087u, and an electron weighs 0.00055u.


Now, why is the mass of chlorine-35 equal to 34.969? Are there not 17 protons, 18 neutrons, and 17 electrons? I calculated it and it sums to around 35.29. Where did I go wrong? 




You need to account for the energy released when nucleons and electrons come together and form a Cl-35 atom. Its called the Binding Energy.


This sort of equation can help to explain :


(Rest Mass Energy of Individual nucleons,electrons\*) - (Various Binding Energies) = (Rest Mass Energy of Natural)


\*edit


 2