What Particles Are Found in the Nucleus of an Atom
An atom’s nucleus contains protons and, in most atoms, neutrons. These particles are called nucleons. Protons have a positive electric charge, while neutrons have no charge. Electrons occupy the space outside the nucleus and are not part of it.
Protons and neutrons account for nearly all an atom’s mass. They are made of smaller particles called quarks, but protons and neutrons are the particles used to describe the contents of an atomic nucleus. The Encyclopaedia Britannica describes the nucleus as the positively charged central region of an atom, made up of protons and neutrons.
How protons and neutrons differ
| Property | Proton | Neutron |
|---|---|---|
| Electric charge | Positive (+1) | None (0) |
| Location in an atom | Nucleus | Nucleus |
| Mass | About 1 atomic mass unit | About 1 atomic mass unit, slightly heavier |
| Determines | Element identity | Isotope |
The Encyclopaedia Britannica entry on the proton describes it as a positively charged particle in atomic nuclei. Its neutron entry describes a neutral particle with a mass close to that of a proton.
The most common form of hydrogen is a familiar exception to the usual presence of neutrons. Its nucleus contains a single proton and no neutrons.
How particle counts define elements and isotopes
An element’s identity depends on its number of protons, also called its atomic number. A nucleus with one proton belongs to hydrogen; one with six protons belongs to carbon. A change in proton count means a change in the element.
The number of neutrons can differ among atoms of the same element. Atoms with the same number of protons but different numbers of neutrons are called isotopes. For example, hydrogen has isotopes with one proton and zero, one, or two neutrons. These are called ordinary hydrogen, deuterium, and tritium, respectively. They are all hydrogen because each has one proton.
The mass number is the total number of protons and neutrons in a nucleus. For example, a carbon nucleus with six protons and six neutrons has a mass number of 12. Mass number counts particles; it is not the same as an isotope’s measured atomic mass.
What holds the nucleus together?
Protons repel one another because they carry the same positive charge. At the very short distances inside a nucleus, the strong nuclear force attracts nucleons and binds the nucleus together. Neutrons add to this binding without adding electric repulsion.
A nucleus is stable when its arrangement of protons and neutrons remains intact. Some nuclei are unstable and decay, releasing particles or energy. The details depend on the nucleus. In beta-minus decay, for example, a neutron changes into a proton and an electron is emitted. That electron is produced during the decay; it is not stored in the nucleus.
The Encyclopaedia Britannica describes nuclear binding as the force that holds nucleons together despite the electrical repulsion between protons.
Why electrons are not in the nucleus
Electrons are part of an atom, but they are outside its nucleus. They occupy the surrounding electron cloud, while the nucleus forms the atom’s small central core. An electron emitted during nuclear decay is a product of the process, not one of the nucleus’s usual structural particles.
Key Takeaways
- Atomic nuclei contain protons and, in most atoms, neutrons. These particles are called nucleons.
- Protons have a positive charge; neutrons have no electric charge.
- The number of protons defines an element. Different neutron counts produce isotopes of that element.
- The strong nuclear force binds nucleons together over short distances.
- Electrons belong to the atom but remain outside the nucleus.
