The nuc of an atom is the very dense region, consisting of nucle(protons and neutrons), at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nuc with a very small contribution from the orbiting electrons.

The diameter of the nuc is in the range of 1.6 fm (1.6 × 10−15 m) (for a proton in light hydrogen) to about 15 fm (for the heaviest atoms, such as uranium). These dimensions are much smaller than the size of the atom itself by a factor of about 23,000 (uranium) to about 145,000 (hydrogen).

The branch of physics concerned with studying and understanding the atomic nuc, including its composition and the forces which bind it together, is called nuc lear physics.