A/An ________ is the smallest chemical unit of matter.

A) atom
B) element
C) proton
D) neutron

Atom

The basic unit of a chemical element, consisting of a nucleus of protons and neutrons, and surrounded by electrons.

Element

is a substance that cannot be broken down into simpler substances by chemical means, consisting of atoms with the same number of protons.

Proton

A subatomic particle found in the nucleus of an atom, carrying a positive electric charge and contributing to the atomic number and mass of the atom.