Quarkie

Meet Quarkie, a proton with three quarks inside and one neat charge outside.

Protons and neutrons are not elementary. Each is built from three quarks, held together by the strong force. A proton contains two up quarks, each with charge +2/3 of the elementary charge, and one down quark with charge −1/3. Murray Gell-Mann and George Zweig independently proposed quarks in 1964, and electron-scattering experiments at SLAC in the late 1960s found point-like particles inside the proton, work recognised by the 1990 Nobel Prize in Physics.

Adding the charges: 2/3 + 2/3 − 1/3 = 3/3 = +1, exactly the proton's charge. A neutron is one up and two down quarks: 2/3 − 1/3 − 1/3 = 0.

The quark model explains why there are so many related particles, and it is a core part of the Standard Model used to design and interpret experiments at particle colliders.

Two honest limits. Quarks have never been isolated on their own; they are always confined inside particles. And the three quarks' own masses make up only about one percent of the proton's mass; most of it comes from the energy of the strong force that binds them.