In the context of Class 12 physics, the study of atomic nuclei involves understanding the structure, properties, and behavior of the nucleus of an atom. Here are some key points:

1. **Nuclear Structure:** The nucleus is composed of protons and neutrons, which are collectively called nucleons. Protons are positively charged, while neutrons have no charge (they are neutral).

2. **Atomic Number and Mass Number:** The number of protons in the nucleus determines the element’s atomic number. The sum of protons and neutrons gives the mass number.

3. **Isotopes:** Atoms of the same element with different numbers of neutrons are called isotopes. They have the same atomic number but different mass numbers.

4. **Nuclear Forces:** The strong nuclear force holds the nucleus together against the electromagnetic repulsion between positively charged protons. This force is very strong but acts over a very short range.

5. **Nuclear Stability:** Nuclei with certain numbers of protons and neutrons are more stable than others. Nuclei that are too heavy or too light tend to be unstable and can undergo radioactive decay.

6. **Radioactivity:** Some nuclei are unstable and undergo radioactive decay, emitting particles and/or energy in the process. The three main types of radioactive decay are alpha decay, beta decay, and gamma decay.

7. **Half-Life:** The half-life of a radioactive substance is the time taken for half of the radioactive nuclei in a sample to decay. It is a characteristic property of each radioactive isotope.

8. **Nuclear Reactions:** Nuclear reactions involve changes in the nucleus of an atom and can result in the transformation of one element into another. Examples include nuclear fission and nuclear fusion.

Studying atomic nuclei is crucial for understanding nuclear physics, radioactivity, and nuclear energy, among other topics.