As far as we know to date, electrons are truly fundamental particles, but protons and neutrons are each composed of different combinations of up and down quarks. The force holding a proton together is so strong that scientists were not able to break them apart until 2010 when two beams of protons with a total collision energy of 7 trillion eV (electron volts) collided in the Large Hadron Collider under France and Switzerland (SOURCE). This is why we state that protons and neutrons are indivisible for the purposes of chemistry, where the energies that we deal with might range from a few eV to a few dozen eV, not 7 trillion eV.