What Particles Are Found in the Nucleus of an Atom? A Clear Guide

The nucleus of an atom contains two types of particles: protons and neutrons. These two particles are collectively called nucleons. Protons carry a positive electric charge, while neutrons are electrically neutral. Together, they account for nearly all the mass of an atom.

The Two Main Particles: Protons and Neutrons

Protons

A proton is a stable subatomic particle with a positive charge of +1 elementary charge. Its mass is approximately 1.6726 × 10⁻²⁷ kilograms, about 1,836 times the mass of an electron. The number of protons in an atom’s nucleus defines the element. For example, every hydrogen atom has exactly one proton, while every carbon atom has six. This number is called the atomic number International Union of Pure and Applied Chemistry (IUPAC).

Neutrons

A neutron is slightly heavier than a proton, with a mass of about 1.6749 × 10⁻²⁷ kilograms. It has no net electric charge. Neutrons are essential for holding the nucleus together. Without them, the positive charges of protons would repel each other so strongly that the nucleus would fly apart. The number of neutrons in an atom can vary for a given element, producing different isotopes. For instance, carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons.

How Protons and Neutrons Are Held Together

The nucleus is an incredibly dense region. To keep positively charged protons from repelling each other, a powerful force is needed: the strong nuclear force, or strong interaction.

The strong nuclear force acts between all nucleons at very short distances, roughly the size of a nucleus. It is about 100 times stronger than the electromagnetic force that causes protons to repel each other. However, it only works over distances of about 1 to 3 femtometers (1 fm = 10⁻¹⁵ meters). If a nucleus gets too large, the strong force cannot reach across its entire volume, and the nucleus becomes unstable. This is why heavy elements like uranium have many neutrons. The extra neutrons add more strong-force attraction without adding more repulsive charge, helping to stabilize the nucleus.

Are There Other Particles in the Nucleus?

The simple answer is no, not as permanent residents. The nucleus contains only protons and neutrons. However, a more complete picture involves two important details.

Quarks: The Building Blocks of Nucleons

Protons and neutrons are not fundamental particles. Each is made of three smaller particles called quarks. A proton consists of two “up” quarks and one “down” quark. A neutron consists of two “down” quarks and one “up” quark. Quarks are held together by another force carrier called the gluon. So while quarks are inside the nucleus, they are always bound inside protons and neutrons. They are not free particles floating in the nucleus CERN.

Transient Particles

During high-energy collisions or radioactive decay, other particles can appear briefly inside the nucleus. For example, in beta decay, a neutron can transform into a proton, releasing an electron and an antineutrino. These particles are not permanent parts of the nucleus. They are created and ejected during the process.

A Quick Worked Example: The Helium Nucleus

Consider a helium-4 atom. Its nucleus contains:

• 2 protons (atomic number 2)
• 2 neutrons (mass number 4 minus atomic number 2)

The two protons give the nucleus a total charge of +2. The two neutrons add mass without charge. The strong nuclear force between all four nucleons holds the nucleus together. This configuration is extremely stable. Helium-4 is the second most abundant element in the universe, after hydrogen.

Nuclear Stability and the Neutron-to-Proton Ratio

Not every combination of protons and neutrons is stable. For light elements, the most stable nuclei have roughly equal numbers of protons and neutrons. For example, oxygen-16 has 8 protons and 8 neutrons. As elements get heavier, the number of neutrons needed for stability increases faster than the number of protons. This is because the electromagnetic repulsion between protons grows with the square of the number of protons, while the strong nuclear force only grows linearly with the number of nucleons.

Uranium-238 has 92 protons and 146 neutrons. That is a neutron-to-proton ratio of about 1.59. This high ratio is necessary to overcome the strong repulsion between the 92 protons. Even so, uranium-238 is not perfectly stable. It undergoes alpha decay, emitting a helium nucleus to become thorium-234.

How We Know What Is in the Nucleus

Scientists did not always know that the nucleus contains protons and neutrons. The discovery came through a series of experiments.

Rutherford’s Gold Foil Experiment (1911)

Ernest Rutherford fired alpha particles at a thin sheet of gold foil. Most particles passed straight through, but a small number bounced back. Rutherford concluded that atoms have a small, dense, positively charged core. He called it the nucleus.

Chadwick’s Discovery of the Neutron (1932)

James Chadwick bombarded beryllium with alpha particles and observed a new type of radiation that could knock protons out of paraffin wax. This radiation was not affected by electric or magnetic fields, meaning it had no charge. Chadwick correctly identified it as a neutral particle with roughly the same mass as a proton. He called it the neutron.

Deep Inelastic Scattering (1960s-1970s)

Later experiments at the Stanford Linear Accelerator Center fired high-energy electrons at protons and neutrons. The way the electrons scattered revealed that protons and neutrons contain smaller, point-like constituents. These constituents were named partons and later identified with quarks.

Key Takeaways

• The nucleus of an atom contains only two types of permanent particles: protons (positively charged) and neutrons (neutral).
• The number of protons determines the element; the number of neutrons determines the isotope.
• Protons and neutrons are held together by the strong nuclear force, which is much stronger than electromagnetic repulsion but acts only at very short distances.
• Protons and neutrons are themselves made of smaller particles called quarks, but quarks are never found free in the nucleus.
• Other particles (like electrons or pions) can appear briefly during nuclear reactions, but they are not permanent parts of the nucleus.