The atom, the fundamental unit of matter, is built around a dense, central structure known as the nucleus. This nucleus occupies an incredibly small volume compared to the atom’s overall size. Despite its minute dimensions, the nucleus contains nearly all of the atom’s mass, giving it a profound density. The nucleus is the source of an element’s identity and is composed of two primary types of subatomic particles collectively called nucleons: protons and neutrons.
The Nucleons: Protons and Neutrons
The proton carries a single positive electrical charge and fundamentally defines an element. Protons are relatively heavy, possessing a mass of approximately one atomic mass unit (amu). Because they are positively charged, protons naturally repel one another due to the electromagnetic force, which makes their co-existence within the tiny nucleus a significant physical puzzle.
The second type of nucleon is the neutron, which is electrically neutral, carrying no net charge. The neutron’s mass is almost identical to that of the proton, also contributing one atomic mass unit to the total nuclear mass. The presence of neutrons helps to moderate the repulsive forces between the positively charged protons. The combined mass of the protons and neutrons accounts for virtually the entire mass of the atom, as electrons have a negligible mass.
Defining Atomic Identity and Mass
The number of protons within the nucleus is the defining feature of any element and is known as the atomic number (\(Z\)). Every atom of a specific element contains the exact same number of protons; changing the number of protons immediately changes the element itself. For instance, an atom with six protons is carbon, and an atom with eight protons is oxygen.
The total count of both protons and neutrons in the nucleus is termed the mass number (\(A\)), which is a whole-number approximation of the atom’s mass in atomic mass units. While the proton count is fixed for an element, the number of neutrons can vary, leading to the existence of isotopes. Isotopes are atoms of the same element that have an identical number of protons but a different number of neutrons, resulting in different mass numbers.
For example, hydrogen most commonly has one proton and zero neutrons, giving it a mass number of 1. The isotope deuterium has one proton and one neutron (mass number 2), while tritium has one proton and two neutrons (mass number 3). These different neutron counts give isotopes distinct nuclear properties, even though their chemical behavior remains almost the same.
The Strong Nuclear Force and Stability
A nucleus containing multiple protons should theoretically fly apart due to the electromagnetic repulsion between their positive charges. The existence of stable nuclei is explained by the strong nuclear force, a separate, fundamental interaction. This force is a powerful attraction that acts between all nucleons—protons and neutrons alike.
The strong force is vastly more intense than the electromagnetic force, but it is extremely short-ranged, only effective across the tiny distances within the nucleus. This short range means that as a nucleus gets larger, the strong force struggles to hold the outermost protons together against the long-range electromagnetic repulsion. The balance between the powerful, short-range strong force and the weaker, long-range electromagnetic repulsion determines the stability of a nucleus.
Processes of Nuclear Change
Although many nuclei are stable, the balance of forces can be disrupted, leading to changes in the nucleus’s composition or energy state. One process is radioactive decay, where an unstable nucleus spontaneously emits particles or energy to achieve a more stable configuration. This results in the transformation of a parent isotope into a daughter isotope, which may be a different element entirely.
Two other major processes involve induced changes to the nucleus, both releasing enormous amounts of energy. Nuclear fission involves the splitting of a heavy nucleus, such as uranium, into two or more smaller nuclei, typically initiated by a neutron. Conversely, nuclear fusion is the combining of two light nuclei, such as forms of hydrogen, to form a single, heavier nucleus, a process that powers the sun. Both fission and fusion reactions release excess energy as heat and radiation.