The core of every atom, the fundamental unit of matter, is a small and dense region known as the nucleus. This central structure holds virtually all of the atom’s mass, despite taking up only about one ten-trillionth of the atom’s total volume. If an entire atom were scaled up to the size of a baseball stadium, its nucleus would be no larger than a baseball at the center. This positively charged core is surrounded by a vast, comparatively empty space where negatively charged electrons orbit, held in place by the electrostatic force. The nucleus determines the atom’s identity and stores the immense energy that governs nuclear reactions.
The Components of the Nucleus
The atomic nucleus is composed of two types of subatomic particles collectively called nucleons: protons and neutrons. Protons carry a single positive electrical charge, giving the nucleus its overall positive charge. Neutrons are electrically neutral. Both particles have nearly the same mass, which is approximately 1,836 times greater than the mass of an electron. Because of this mass equality, both protons and neutrons contribute almost equally to the atom’s total atomic mass.
How the Nucleus Defines the Element
The number of protons within the nucleus determines the identity of an atom, establishing its atomic number (Z). For example, any atom containing six protons is carbon, while an atom with eight protons is oxygen. Changing the number of protons immediately changes the element itself. The number of neutrons, however, can vary within an element, creating different forms called isotopes. Isotopes of the same element have the same chemical properties because their electron configurations are unchanged. The difference in neutron count only affects the atomic mass and the nuclear stability of the atom.
The Force That Binds the Nucleus
How do positively charged protons remain packed together in such a tiny space, given that like charges strongly repel each other through the electromagnetic force? The answer lies in the strong nuclear force, the most powerful of the four fundamental forces in nature. This force acts as a powerful attraction between all nucleons—protons and neutrons alike—that is strong enough to overcome the electromagnetic repulsion between protons. The strong nuclear force is a short-range force; its attractive power drops to nearly zero at distances greater than about 2.5 femtometers, limiting its effects to within the nucleus.
The energy associated with this attraction is known as the nuclear binding energy. This binding energy is the amount of energy required to separate the nucleus into its individual protons and neutrons. The formation of a nucleus results in a small amount of mass being converted into energy, a phenomenon described by Einstein’s equation, E=mc². Nuclear binding energies are millions of times greater than the energy that holds electrons to the atom.
When the Nucleus Changes
A nucleus is considered unstable, or radioactive, when the balance of forces between the protons and neutrons is off. To reach a more stable state, the nucleus undergoes radioactive decay, emitting particles or energy as radiation. Since these processes change the number of protons, the parent atom transforms into a different chemical element. Humans have also learned to manipulate the nucleus to release energy for practical applications through fission and fusion.
Radioactive Decay
Common types of decay include alpha decay, where the nucleus ejects a particle consisting of two protons and two neutrons, and beta decay, where a neutron converts into a proton or vice versa.
Nuclear Fission
Nuclear fission is the process where a large, unstable nucleus, such as Uranium-235, is split into two or more smaller nuclei, releasing a large amount of energy and extra neutrons that can sustain a chain reaction.
Nuclear Fusion
Conversely, nuclear fusion is the process of combining two light nuclei, such as isotopes of hydrogen, to form a single heavier nucleus. Fusion is the reaction that powers the sun and releases greater amounts of energy than fission, making it a major focus of energy research.