Once Proton, Neutron, Electron, Magnetron went to a Bar to have fun. All of them individually ordered for drinks and enjoyed. The waiter gave the bills to only 3 people, and one of them did not get the bill. Who was that ?

This is a physics joke - neutrons have no electrical charge (they're neutral), so in a bar scenario, 'getting charged' (receiving a bill) doesn't apply to them. Protons are positively charged, electrons are negatively charged, and magnetrons aren't subatomic particles but vacuum tubes. The humor relies on the double meaning of 'charged'.