Reality → Matter → Atoms → Nucleus
Protons (with a positive electric charge) and neutrons (no charge) are squeezed tightly together to form the atomic nucleus. To overcome the repellent electrostatic force of the protons, the nuclear force holds the nucleus together [1] . The number of protons determines the element and the number of neutrons determines the isotope of the element and the stability of its nucleus [2] . The atomic number indicates the number of protons (or positive electrical charges), while the mass number is the sum of atomic number and neutron number and indicates the nucleus mass [3] . Protons and neutrons are of about the same size (2.5 × 10-25 m) and have about the same mass (1.7 × 10-27 kg) [4] . Both particles are not elementary; they are thought to consist of quarks [5] .
The nuclear force is a residual effect of the strong interaction of quarks that make up protons and neutrons (see also Note 5). The nuclear force is at the core of nuclear physics, which has applications in nuclear energy (see Fission and Fusion) and nuclear medicine (see also Isotopes).
The number of neutrons is generally higher than the number of protons and provides stability of the nucleus. However, nuclei with more than 82 protons (i.e., heavier than lead) are unstable and show radioactive decay despite a higher share of neutrons (for the artificial splitting of heavy atoms, see Fission).
The unified atomic mass unit (symbol u, or Da for dalton) is defined as 1/12 of the mass of a carbon 12 atom and is equivalent to about 1.66 x 10-27 kg (or 1 gram divided by the Avogadro constant). The unit is widely used in chemistry and named in honor of Dalton and his work in developing the atomic theory based on Stoichiometry.
Quarks, not protons and neutrons, are believed to be the principal elementary constituents of matter. In a fascinating theory based on symmetry (quantum chromodynamics), protons and neutrons derive their masses principally from the interaction of quarks and gluons . According to the concept of mass-energy equivalence, about 99 % of the quark mass is ascribed to the binding energy of gluons, and only 1 % to quarks by themselves (see also Major discoveries, Note 4).