What is the basic unit of matter?

The basic unit of matter is an atom. Atoms are the fundamental building blocks of all substances and consist of a nucleus made up of protons and neutrons, surrounded by electrons that occupy various energy levels around the nucleus. Atoms can combine to form molecules, which are made up of two or more atoms bonded together. While molecules are essential in understanding chemical interactions, they are not considered the most basic unit of matter since they are composed of atoms.

Ionic compounds refer to substances formed by the electrostatic attraction between ions, which are themselves made up of atoms, thus they also do not represent the basic unit of matter. An element, in the context of chemistry, is a pure substance that consists of only one type of atom; however, the term "element" does not specifically denote the individual unit of matter. In essence, while elements are composed of atoms, it is the atom that fundamentally defines the smallest unit of matter in its purest form.