the tiny, crowded rooms where atoms keep almost all their mass and none of their manners.
means nuclei are the dense clusters of protons and neutrons at the center of atoms, holding nearly all the atom's mass in a space a hundred thousand times smaller than the atom itself.
from from latin nucleus, meaning kernel or little nut, first borrowed by botanists for seed cores before physicists in the early 1900s repurposed it for the atom's dense center after rutherford's gold foil experiment revealed something small and heavy hiding inside mostly empty space.
uranium-235 — splits in fission reactors and bombs, discovered 1938
hydrogen nucleus — just one proton, simplest nucleus in the universe
iron-56 — most stable nucleus, peak of the binding energy curve
deuterium — proton plus neutron, fuels fusion in stars and bombs