The atomic nucleus is the tiny, dense core of an atom that contains protons and neutrons held together by the strong force. Collectively, the protons and neutrons in the nucleus are called nucleons. The number of protons in the atomic nucleus identifies the element of an atom. Knowing the element, the number of neutrons in the nucleus identifies its isotope.
- The atomic nucleus consists of protons and neutrons.
- The nucleus has a positive electrical charge.
- Nuclear composition determines an atom’s element (number of protons) and isotope (number of neutrons).
- The nucleus is very small and dense. It accounts for nearly all of the atomic mass, but very little of its volume.
The word nucleus comes from the Latin word nucleus, which means “kernel” or “nut.” Michael Faraday referred to the center of the atom as a nucleus in 1844 and Rutherford used the term in 1912. However, other scientists did not immediately adopt it and referred to the atomic nucleus as a kernel for several years.
Ernest Rutherford’s discovery of the atomic nucleus in 1911 traces its roots in the 1909 Geiger-Marsden gold foil experiment. The gold foil experiment involved shooting alpha particles (helium nuclei) at a thin sheet of gold. If the alpha particles easily passed through the gold, it would support J. J. Thomson’s “plum pudding model” of the atom, with an atom consisting of intermingling positive and negative charge. But, many of the alpha particles bounced away from the foil, meaning atoms consist of separate regions of positive and negative charge.
The 1932 discovery of the neutron led to a better understanding of the nature of the atomic nucleus. Dmitri Ivanenko and Werner Heisenberg proposed a model of the atom with a positively-charged nucleus surrounded by a cloud of negatively charged electrons.
What Does the Atomic Nucleus Contain?
The atomic nucleus consists of protons and neutrons. Protons and neutrons are made of subatomic particles called quarks. The quarks exchange another type of subatomic particle (gluons). This exchange is the strong force that binds the particles together within the nucleus. The strong force acts over a short range, but it is more powerful than the electrostatic repulsion between positively-charged protons.
While we normally think of protons and neutrons as particles, they also have properties of waves. Because protons and neutrons have different quantum states, they can share the same space wave function. In effect, two protons, two neutrons, or a proton and a neutron form a nucleon, with the two particles sharing the same space.
Although not observed in nature, high-energy physics experiments sometimes report a third baryon, called a hyperon. A hyperon is a subatomic particle much like a proton or a neutron, except contains one or more strange quarks.
Usually, the nucleus does not contain electrons because they scatter away from the atomic core. However, the wave function describing the probability of finding an electron in any particular region does pass through the nucleus.
How Big Is the Atomic Nucleus?
The atomic nucleus is extremely tiny, yet very dense. It accounts for less than one ten-trillionth of the volume of an atom, but about 99.9994% of an atom’s mass. To put it another way, an atom the size of a football field has a nucleus the side of a pea.
The average size of an atomic nucleus ranges between 1.8 × 10 −15 m (hydrogen) and 11.7 × 10 −15 m (uranium). In contrast, the average size of an atom ranges between 52.92 x 10-12 m (hydrogen) and 156 x 10-12 m (uranium). This is a difference by a factor of about 60,000 for hydrogen and 27,000 for uranium.
What Is the Shape of the Atomic Nucleus?
Typically, the shape of the atomic nucleus is round or ellipsoid. However, other shapes occur. Here are the nucleus shapes observed to date:
- Deformed prolate (like a rugby ball)
- Deformed oblate (like a discus)
- Triaxial (like a combination of a rugby ball and a discus)
- Halo-shaped (a small core surrounded by a halo of excess protons or neutrons)
An atom diagram usually depicts the nucleus as a cluster of equal-sized protons and neutrons with orbiting electrons. Of course, this is an oversimplification. There are multiple models of the atomic nucleus:
- Cluster model: The cluster model includes the one you see in diagrams, with protons and neutrons grouped together. Modern cluster models are more complex, with two- and three-body clusters forming more complex nuclear structures.
- Liquid drop model: In this model, the nucleus acts as a rotating liquid drop. This model explains the size, composition, and binding energy of nuclei, but does not explain the stability of “magic numbers” of protons and neutrons.
- Shell model: This model views the structure of nucleons much like the structure of electrons, where nucleons occupy orbitals. Placing protons and neutrons in orbitals successfully predicts magic number because the models allow for stable configurations. Shell models break down when discussing nuclear behavior outside of closed nuclear shells.
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