What is the smallest unit of matter and the basic building block of all substances?

The smallest unit of matter and the basic building block of all substances is the atom. Atoms are the fundamental components that make up all matter, including gases, liquids, and solids. Each atom consists of a nucleus, containing protons and neutrons, surrounded by a cloud of electrons.

Atoms combine in various ways to form different substances, such as molecules and compounds. A molecule is formed when two or more atoms bond together, while a compound is a specific type of molecule that contains at least two different elements. Ions are charged atoms or molecules that have lost or gained one or more electrons, but they are not the simplest form of matter.

Understanding that atoms are the foundational units of matter helps clarify their role in forming all physical substances and how they interact with one another to create the diverse materials found in the universe.