What is the simplest form of matter?

The simplest form of matter that has a unique set of properties and is cannot be broken down into simpler substances by chemical means is called an Element.

An Atom is the simplest form of an element.

The simplest form of some elements is a Molecule. When an element consists of two identical atoms it is called a diatomic molecule.

A substance that contains two or more elements chemically combined in a fixed proportion is called a Compound.