Short answers, pulled from the story.
The strong interaction exerts a force roughly one hundred times stronger than electromagnetism at distances of 10 to the minus 15 meters. It reaches ten to the sixth power times the strength of the weak interaction and ten to the thirty-eighth power times that of gravity.
Murray Gell-Mann and George Zweig independently proposed that baryons like protons and neutrons contained smaller elementary particles in 1964. Zweig called these particles aces while Gell-Mann named them quarks.
Experiments searching for free quarks consistently fail because the force remains constant when pulling two quarks apart beyond about 0.8 femtometers. This unyielding force creates enough energy to form new quark-antiquark pairs before isolation can occur.
A residual force emerges between colorless hadrons by transmitting virtual mesons like pions and rho mesons to hold the nucleus together. This force diminishes rapidly as distance increases following an exponential power law known as the Yukawa potential.
The instability of larger atomic nuclei arises because the attractive residual force decreases faster than the repulsive electromagnetic force acting between protons. All elements with atomic numbers greater than 82 exhibit this instability due to differences in mass defects resulting from this force.