The central part of an atom is called the nucleus. The atom is the fundamental unit of matter. The nucleus forms the tiny, central core of the atom, holding almost all of its mass within a minuscule volume. For perspective, the nucleus occupies less than one ten-trillionth of the atom’s total space, making the atom mostly empty space. Despite its small size, the nucleus is incredibly dense, possessing a density more than 2 x 10¹⁴ times greater than water.
The Nucleus: Composition of Subatomic Particles
The atomic nucleus is composed of two types of subatomic particles collectively referred to as nucleons: protons and neutrons. Protons carry a single positive electrical charge, while neutrons are electrically neutral. The presence of these positively charged protons gives the entire nucleus a net positive charge.
The total number of protons and neutrons determines the atom’s overall mass number. Protons and neutrons have nearly identical mass, both being thousands of times heavier than the electrons that orbit the nucleus. This mass difference ensures that over 99.9% of an atom’s mass resides within the nucleus.
Determining Atomic Identity and Mass
The number of protons within the nucleus is the defining characteristic of an element, known as the atomic number. If an atom has one proton, it is hydrogen; if it has six protons, it is carbon. Changing the number of protons fundamentally changes the element itself, a process that requires a nuclear reaction. The atomic number also dictates the number of electrons in a neutral atom, which determines its chemical properties.
The mass of an atom is primarily determined by the total count of nucleons, known as the mass number. While the proton count fixes the element’s identity, the number of neutrons can vary for atoms of the same element. Atoms that share the same number of protons but have differing numbers of neutrons are known as isotopes. For instance, Carbon-12 and Carbon-14 are both carbon because they each have six protons, but they differ in their neutron count and atomic mass.
The Forces That Maintain Nuclear Stability
A fundamental challenge exists within the nucleus because multiple positively charged protons are packed tightly together in a tiny space. The electromagnetic force dictates that like charges repel one another, meaning the protons should naturally push the nucleus apart. To overcome this powerful electrostatic repulsion, a far stronger force must hold the nucleus together.
This binding mechanism is the Strong Nuclear Force, the strongest of the four fundamental forces of nature. The strong force is powerful, but it has an extremely short range, acting only over distances of a few femtometers, roughly the diameter of a nucleon. The strong force works between any pair of nucleons—proton-proton, neutron-neutron, and proton-neutron—binding them into a stable configuration.
In large nuclei, the long-range electromagnetic repulsion between protons can begin to overcome the short-range strong force. This imbalance can lead to nuclear instability, causing the nucleus to undergo radioactive decay.