To find the binding energy of a nucleus , it does not involve the electrons . If an atom mass is given it is inclusive of all the electron mass. If nuclei mass is given it does not include electrons. Therefore it is only logicial to take away electrons mass from the atom mass so that the nuclei mass can be obtained. Example: Binding energy of C is [12x mass of proton + 12 x mass of neutron - mass of C nuclei] x c*2.The mass of C atom is 12u . Therefore the mass of C nuclei is
12 u -12 x mass of electron.
