Which of the following is not found in the nucleus of an atom?
electron
proton
quark
neutron

The correct answer is the first option. Electron is not found in the nucleus of an atom. The sub-atomic particles of an atom are the proton, electron and the neutron. An electron has a charge of -1 and a smaller mass than a proton. Proton has the same mass with the neutron. The ratio between the mass of a proton and an electron is about 2000. An electron has an equal value but negative charge with the proton.