The smallest particle of an element that retains its chemical properties is an atom.
An atom is the basic unit of a chemical element. It contains a dense nucleus made of protons and, usually, neutrons, surrounded by electrons in a cloud of quantum-mechanical states. The number of protons defines the element: every carbon atom has six protons, while every oxygen atom has eight.
Atoms are extremely small. A typical atomic scale is about one angstrom, or 10⁻¹⁰ metre, although atomic size varies with the element and with how the atom is bonded. Most of an atom’s volume is the electron cloud, while nearly all its mass is concentrated in the nucleus.
A frequent mix-up is choosing a molecule. A molecule consists of two or more atoms chemically bonded together, such as H₂O. Protons and neutrons are smaller components inside the nucleus, but they do not retain the chemical identity of an element. Ions are atoms or molecules with a net electric charge; gaining or losing electrons changes their charge and chemistry, but an isolated neutral atom remains the standard answer to this definition.