Why is the smallest particle of iron an atom?

Answer 1

It is an atoms chemical properties that determine whether it is iron or manganese or aluminium. These properties are (to a first approximation) controlled by the number of electrons in the outermost shell.

A single atom is the smallest unit that has the chemical properties.

Anything smaller is electrons or protons, or neutrons ... .

Answer 2

The smallest particle of iron is an atom because iron is a chemical element, and atoms are the fundamental units that make up elements. In the case of iron, each iron atom contains a specific number of protons, neutrons, and electrons that define its atomic structure.