It was dalton who gave the theory that atoms are the smallest unit there can be and it can't be divided further. If we see now, we know atoms are further made if smaller units some of them having independend existance. So, considering atoms as basic unit and smallest units is wrong.

Atoms: The Basic Unit of Matter

Atoms are considered the basic unit of matter and the smallest units for several reasons. To understand why, let's delve into the structure and characteristics of atoms.

1. Definition and Composition of Atoms
- Atoms are the fundamental building blocks of matter.
- They consist of three main subatomic particles: protons, neutrons, and electrons.
- Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge.
- The protons and neutrons are located in the nucleus at the center of the atom, while electrons orbit around the nucleus in electron shells.

2. Size of Atoms
- Atoms are incredibly small, with sizes ranging from 0.1 to 0.5 nanometers (nm).
- To put this into perspective, a sheet of paper is about 100,000 nanometers thick.
- Due to their minuscule size, atoms cannot be seen with the naked eye and require advanced scientific techniques, such as electron microscopes, to be observed.

3. Stability of Atoms
- Atoms are considered stable when they have a balanced number of protons and electrons, resulting in a neutral charge.
- The number of protons determines the atomic number, which defines the element.
- For example, hydrogen has one proton, helium has two, and oxygen has eight.
- The arrangement and number of electrons in the electron shells determine the atom's chemical properties and behavior.

4. Combination of Atoms
- Atoms have the ability to combine with other atoms to form molecules and compounds.
- These combinations occur through chemical reactions, where atoms share or transfer electrons with one another.
- The resulting molecules and compounds exhibit unique properties and characteristics that differ from their constituent atoms.

5. Indivisibility of Atoms
- Atoms are considered indivisible because they cannot be broken down into smaller particles by chemical means.
- While subatomic particles exist within atoms, they remain bound together by strong forces, such as the electromagnetic force and the strong nuclear force.
- Breaking these bonds requires immense amounts of energy, often achieved through nuclear reactions.

6. Energy Levels and Quantum Mechanics
- Atoms also exhibit quantized energy levels, as described by quantum mechanics.
- Electrons occupy specific energy levels or orbitals, and they can transition between these levels by absorbing or emitting energy in discrete quantities.
- This phenomenon explains various atomic phenomena and spectroscopy.

Conclusion
In summary, atoms are considered the basic unit of matter and the smallest units due to their fundamental composition, small size, stability, ability to combine, indivisibility, and their behavior governed by quantum mechanics. Understanding the structure and properties of atoms is crucial in comprehending the nature of matter and the universe around us.