Atoms are the basic building blocks of matter, composed of subatomic particles. Here’s a brief overview:

1. **Structure:** Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in orbitals.

2. **Protons:** Positively charged particles in the nucleus, each with a mass of approximately 1 atomic mass unit (u).

3. **Neutrons:** Neutral particles in the nucleus, with a mass similar to that of protons.

4. **Electrons:** Negatively charged particles that orbit the nucleus in specific energy levels or shells.

5. **Element:** Atoms of the same element have the same number of protons. The number of protons in an atom determines its atomic number and defines the element.

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

7. **Electron Shells:** Electrons occupy specific energy levels or shells around the nucleus. The innermost shell can hold up to 2 electrons, while the next shells can hold more.

8. **Valence Electrons:** Electrons in the outermost shell are called valence electrons. They determine the chemical properties of an element.

9. **Chemical Bonding:** Atoms can form bonds by sharing, gaining, or losing electrons to achieve a stable electron configuration.

10. **Molecules:** When atoms bond together, they form molecules, which are the smallest units of compounds.

Understanding the structure and behavior of atoms is essential for understanding chemistry and the physical world around us.