Nucleus:
The nucleus is the central part of an atom that contains protons and neutrons. It is the densest part of an atom and carries a positive charge due to the presence of protons. The number of protons in the nucleus determines the atomic number of an element, while the total number of protons and neutrons determines the mass number.

Nuclei:
The term "nuclei" is the plural form of the word "nucleus." It refers to multiple nuclei of atoms. Each atom has a nucleus, and when discussing a collection of atoms, we use the term "nuclei" to refer to the multiple central parts of those atoms.

Nucleons:
Nucleons are the subatomic particles present in the nucleus of an atom. They include both protons and neutrons. Protons carry a positive charge, while neutrons do not carry any charge. Nucleons are the building blocks of atomic nuclei and are held together by the strong nuclear force.

Differences:
1. Definition:
- Nucleus: The central part of an atom containing protons and neutrons.
- Nuclei: The plural form of nucleus, referring to multiple central parts of atoms.
- Nucleons: Subatomic particles (protons and neutrons) present in the nucleus.

2. Singularity vs. Plurality:
- Nucleus is singular, representing the central part of an individual atom.
- Nuclei is plural, referring to multiple nuclei of different atoms.
- Nucleons represent the individual subatomic particles present in the nucleus.

3. Composition:
- Nucleus contains protons and neutrons, which determine the atomic properties of an element.
- Nuclei collectively represent the central parts of multiple atoms, each composed of protons and neutrons.
- Nucleons refer to both protons and neutrons, the subatomic particles present within the nucleus.

4. Charge:
- Nucleus carries a positive charge due to the presence of protons.
- Nuclei collectively also carry a positive charge based on the total number of protons in all the nuclei.
- Nucleons individually carry positive charge (protons) or no charge (neutrons).

5. Interactions:
- Nucleus interacts with electrons in the electron cloud to form atoms and participate in chemical reactions.
- Nuclei interact with other nuclei through various forces, such as electromagnetic and nuclear forces.
- Nucleons interact with each other through the strong nuclear force, which holds the nucleus together.

In summary, the nucleus refers to the central part of an atom, nuclei represent multiple central parts of atoms, and nucleons are the individual subatomic particles (protons and neutrons) within the nucleus.