A proton weighs 1.007276 amu , a neutron weighs 1.008665 amu, and an electron weighs 5.486×10−4 amu. What is the expected mass of a beryllium-9 nucleus, based on the total mass of its protons and neutrons?
This question is similar to "you buy 5 lemons at $0.33 each and 5 oranges at $0.45 each. How much change do you have from $5-00 ?#
You have a
The mass of the electron is probably insignificant with respect to the mass of the nuclides. It would not be with respect to the binding energy, which I suppose is the next question.
You might have got a quicker and more sophisticated answer in the physics forum.