Is the smallest chemical unit of a type of pure substance.

The smallest part of an element which is characterised by sharing the chemical properties of an element. or has a property of a chemical element is said to be an atom.

An atom consist of nucleus which is surrounded by electrons i.e. negatively charged particle. Atoms also contains protons i.e. positively charged particle and neutrons i.e. neutral particle. The atomic size depends upon the number of protons, number of neutrons as well on number of electrons.

Hence, atom is the smallest chemical unit of a type of pure substance.

14 days is the half-life of phosphorus-32