Atomic Nuclei are the compact, positively charged central components of atoms, composed primarily of protons and neutrons bound together by the strong interaction. The number of protons defines the atomic number and therefore the chemical element, while the total number of protons and neutrons defines the mass number. Atoms of the same element can contain different numbers of neutrons and are called isotopes. The nucleus occupies only a tiny fraction of an atom's volume but contains almost all of its mass. Its size is typically measured in femtometres, whereas ordinary atomic dimensions are roughly tens of thousands of times larger.
The nucleus is composed of nucleons, meaning protons and neutrons. Protons have positive electric charge, while neutrons have zero net electric charge. Both have spin one half and are composite particles made from quarks bound by the strong interaction. At the nuclear scale, the residual strong interaction between nucleons produces an attractive force over short distances, while proton proton electromagnetic repulsion acts over longer range. Quantum mechanical effects, including the Pauli exclusion principle, strongly influence how nucleons can occupy available nuclear states. These interactions produce discrete nuclear energy levels and a wide variety of ground and excited states.
Atomic nuclei are characterised by quantities including atomic number, mass number, neutron number, nuclear spin, parity, binding energy, magnetic moment and electric quadrupole moment. Nuclear notation identifies a nuclide through its proton and neutron counts, allowing isotopes and isobars to be distinguished. Isotopes have the same atomic number but different neutron numbers, isobars have the same mass number but different atomic numbers, and isotones have the same neutron number but different proton numbers. Nuclear stability depends on the balance of these quantities rather than on proton number alone. Some nuclei are stable over extremely long periods, while others transform through radioactive decay.
Nuclear binding arises because the bound nucleus has lower total energy than the corresponding separated constituents. The associated mass defect corresponds to binding energy through \(E=mc^2\). Binding energy per nucleon varies systematically with nuclear composition, producing the broad stability pattern across the chart of nuclides. Very light nuclei can release energy through fusion, while very heavy nuclei can release energy through fission. The precise stability of an individual nuclide depends on shell effects, pairing, deformation, neutron proton balance and other quantum properties, so simple average binding trends do not completely determine nuclear behaviour.
Atomic nuclei possess excited states analogous in mathematical structure to quantised states in other quantum systems. A nucleus can absorb energy and move to an excited configuration, after which it may return to a lower state by emitting electromagnetic radiation, often in the form of gamma rays. Nuclear transitions can also involve particle emission or other processes when sufficient energy is available. Nuclear spin and angular momentum selection rules constrain allowed transitions, while transition probabilities determine lifetimes and decay characteristics. Long-lived excited states called metastable states can persist significantly longer than ordinary excited states before decaying.
Nuclear stability depends strongly on the relationship between proton and neutron numbers. Stable light nuclei generally have proton and neutron numbers of comparable magnitude, while stable heavier nuclei require proportionally more neutrons because additional neutron mediated nuclear attraction helps offset increasing proton proton electromagnetic repulsion. Beyond the region of stable nuclei, neutron-rich or proton-rich nuclides can undergo radioactive transformations that move their composition towards greater stability. The chart of nuclides therefore provides a map of nuclear species and their decay relationships rather than merely a list of chemical elements.
Atomic nuclei can interact through nuclear reactions when exposed to other particles or radiation. Neutron capture, proton capture, alpha capture, fission and fusion can all transform a nucleus. Reaction thresholds, resonances and cross sections determine how probable particular processes are at given energies. Nuclear reactions can also create isotopes that do not occur naturally in significant quantities. These artificial radionuclides have important medical, industrial and scientific applications, demonstrating that nuclei are dynamic quantum systems whose composition can be changed deliberately under suitable conditions.
The study of atomic nuclei has revealed important distinctions between nuclear and atomic scales. Chemical properties are primarily determined by electron configurations and electromagnetic interactions involving electrons, while nuclear properties depend on proton neutron composition and the strong interaction. A change in neutron number can leave the chemical element unchanged while substantially altering radioactive behaviour, mass and nuclear stability. A change in proton number changes the element itself. Understanding these distinctions is essential for interpreting isotopes, radioactive decay, nuclear reactions, mass spectroscopy, nuclear medicine and the broader structure of matter.
