Where Protons and Neutrons Are Found in the Atom
Protons and neutrons are in the atomic nucleus, the small, dense core at the center of an atom. Electrons occupy the space around the nucleus. Because protons and neutrons are much heavier than electrons, the nucleus contains nearly all of an atom’s mass Encyclopaedia Britannica — Atomic Nucleus Encyclopaedia Britannica — Atom.
A proton has a positive electrical charge. A neutron has no electrical charge. The number of protons in a nucleus determines the atom’s element. The number of neutrons can vary, so atoms of the same element can have different forms, called isotopes.
What the nucleus contains
The nucleus contains protons and, in most atoms, neutrons. These particles occupy a very small space compared with the atom as a whole. As a result, the nucleus is dense, even though it takes up only a small part of the atom’s total volume.
Protons repel one another because they carry positive charges. At the short distances inside a nucleus, the strong nuclear force binds protons and neutrons together. This force does not hold electrons in the nucleus. Electrons occupy regions around it and are attracted to its positive charge.
The nucleus also determines an atom’s atomic number and mass number:
- Atomic number is the number of protons. It identifies the element and is the same for every atom of that element.
- Mass number is the total number of protons and neutrons in a particular nucleus.
To find the neutron count, subtract the atomic number from the mass number:
Number of neutrons = mass number − atomic number
Atomic data references such as the NIST Atomic Data for the Periodic Table list the atomic number for each element. A periodic table may also show an element’s atomic mass, a weighted value based on naturally occurring isotopes. Atomic mass is not always the mass number of a single atom, so use an isotope’s mass number to calculate its neutron count.
One example: carbon-12
Carbon has an atomic number of 6, so every carbon nucleus contains 6 protons. A carbon-12 nucleus has a mass number of 12. Subtract the proton count to find the neutron count:
12 − 6 = 6 neutrons
Carbon-12 has 6 protons and 6 neutrons in its nucleus. Another carbon isotope can have a different number of neutrons while keeping the same 6 protons. The unchanged proton count identifies it as carbon.
Why the particles stay in the nucleus
The strong nuclear force acts over very short distances. Inside the nucleus, it can bind the particles together despite the electrical repulsion between protons. The balance of forces affects which nuclei are stable. Neutrons contribute to nuclear binding without adding positive electrical charge.
A nucleus can change during radioactive decay or a nuclear reaction. These processes can alter the number of protons, neutrons, or both. If the proton count changes, the atom becomes a different element. If only the neutron count changes, it becomes a different isotope of the same element.
Electrons, by contrast, occupy the space around the nucleus. They have a negative charge and much less mass than protons and neutrons. This difference explains why the nucleus accounts for nearly all the atom’s mass, while electrons occupy the region associated with many of its chemical properties.
Key Takeaways
- Protons and neutrons are found in the nucleus at the center of an atom.
- Protons have a positive charge; neutrons have no charge.
- The number of protons determines the element. Isotopes of an element have the same proton count but can have different neutron counts.
- The mass number is the total number of protons and neutrons. Subtract the atomic number from it to find the neutron count.
- The strong nuclear force binds particles in the nucleus, while electrons occupy the space around it.
