It would be an atom or a molecule (made up of more than one atoms)
The smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms is a molecule.
Element
atoms
It is a molecule of the substance.
Molecule
Molecule
An atom is the smallest particle of a substance.
A molecule; the definition is: The smallest particle of a substance that retains the chemical and physical properties of the substance and is composed of two or more atoms; a group of like or different atoms held together by chemical forces.
If the substance is an element the answer is an atom. If the substance is a compound the answer is a molecule.
A molecule is the smallest particle of a compound that still retains its properties. An atom is the smallest particle of an element that still retains its properties.
An atom is the smallest particle of a substance.
For an element, it would be an atom. For a molecular substance, it would be a molecule. If it is an ionic compound, it would be a formula unit.