The matter is composed of invisible, indivisible, innumerable particles called atoms. Atoms are responsible for giving each matter its unique properties and identity. The uniqueness comes from the nature of the atom and its composition. Atoms are composed of three subatomic particles- electron, neutron, and proton. For example, an Oxygen atom has 8 electrons, 8 protons, and 8 neutrons.
The proton and neutron form the atom's nucleus and are surrounded by an electron cloud. Only protons and electrons are charged particles and the neutrons are neutral. The proton's positive charge in the nucleus balances out with the negative charge of the electron in the electron cloud, rendering an atom electrically neutral. Together, they form an atom so tiny, about the size of 10-10m, equivalent to a hundred-millionth of a centimeter.
When atoms combine, they form a molecule. Therefore, a combination of similar atoms creates homoatomic molecules, and dissimilar atoms give heteroatomic molecules.
For example, if two identical H atoms combine, it forms one H2 molecule.
Whereas a combination of two similar H atoms with one dissimilar Oxygen atom forms one water (H2O) molecule.
Many atoms also choose to remain neutral (e.g., He, Ar), while some are explosive in nature (e.g., Na in water).
Therefore, all the solids, liquids, gases, and plasma's peculiar properties are due to the atoms.
Atoms can undergo various nuclear and chemical reactions that can change an atom's identity and properties.