The atomic nucleus is the dense, positively charged center of an atom. Discovered in 1911 by Ernest Rutherford, it occupies an extremely small volume, typically femtometers in diameter, yet contains over 99.9% of the atom’s total mass. This core defines an element and its behavior, despite being surrounded by a much larger electron cloud.
Building Blocks of the Nucleus
The nucleus is composed of subatomic particles called nucleons: protons and neutrons. Protons carry a single positive electric charge (+1e) and have a mass of approximately one atomic mass unit (amu). Neutrons are electrically neutral and have a mass slightly greater than that of protons, also close to one atomic mass unit.
The number of protons in a nucleus is the atomic number (Z), which uniquely identifies an element. For instance, hydrogen atoms have one proton, and helium atoms have two. The total number of protons and neutrons in a nucleus is the mass number (A). Atoms of the same element can have different numbers of neutrons, leading to different isotopes, such as hydrogen, deuterium, and tritium, which all have one proton but varying neutron counts.
The Force That Holds It Together
The strong nuclear force, also called the strong interaction, binds nucleons within the nucleus. This force is powerful, approximately 100 times stronger than the electromagnetic force. Its strength is necessary to overcome the electrostatic repulsion between positively charged protons packed closely together.
The strong nuclear force operates over very short distances, typically 1 to 3 femtometers. At separations below 0.5 femtometers, the force becomes highly repulsive, preventing nucleons from collapsing. However, at distances between 0.5 and 3.0 femtometers, it becomes strongly attractive, reaching maximum attraction around 1.0 femtometer. Beyond approximately 3.0 femtometers, its strength rapidly diminishes to nearly zero, which explains why it does not affect macroscopic objects.
Nuclear Stability and Transformations
Nuclear stability describes which combinations of protons and neutrons form stable nuclei. This stability is largely governed by the neutron-to-proton (N/Z) ratio within the nucleus. For lighter elements (atomic numbers up to 20), stable nuclei typically have an N/Z ratio close to 1:1. As the atomic number increases, the ratio shifts, requiring more neutrons than protons to counteract increasing electromagnetic repulsion. For very heavy elements, this ratio can reach approximately 1.5:1.
When the N/Z ratio falls outside this “band of stability,” nuclei become unstable and undergo radioactive decay to achieve a more stable configuration. For example, nuclei with too many neutrons often undergo beta-minus decay, where a neutron transforms into a proton, emitting an electron. Conversely, nuclei with too many protons may undergo beta-plus decay (positron emission) or electron capture, converting a proton into a neutron. Alpha decay involves the emission of an alpha particle (a helium nucleus of two protons and two neutrons), reducing both the atomic number and mass. Binding energy per nucleon, the energy required to disassemble a nucleus, indicates stability; higher values typically correspond to greater stability.
How Scientists Study the Nucleus
Scientists employ various methods and models to investigate the structure and behavior of the atomic nucleus. Particle accelerators, also known as “atom smashers” or colliders, are key tools. These machines accelerate charged particles, such as protons or atomic nuclei, to speeds approaching the speed of light.
These high-energy particles collide with target nuclei or other particle beams. By analyzing the results, including scattered particles and any new particles produced, scientists gain insights into the forces and constituents within the nucleus. Theoretical models, such as the liquid drop model and the nuclear shell model, also assist in understanding nuclear properties. The liquid drop model treats the nucleus as an incompressible fluid, explaining bulk properties like binding energy and fission. The nuclear shell model, analogous to electron shells in atoms, describes how nucleons occupy distinct energy levels, accounting for specific patterns of nuclear stability.