Which statement is the best definition of an atom?
a. anything that has mass and occupies space.
b. the smallest particle that has the properties of an element.
c. a substance that cannot be broken down chemically all of the properties characteristic of that substance.
d. the smallest unit of a substance that exhibits all of the properties of that substance

The correct option is B.

All elements are made up of atoms. Atom refers to the smallest particle of an element, that possess the properties of that element. Each atom of an element has three sub particles, which are proton, neutron and electron. The proton and the neutron are located inside the nucleus of the atom, while the electron is located outside the nucleus; the electrons are negatively charged and are in continuous motion around the nucleus. Proton is positively charged while neutrons is neutral. It is the electrons that are found in elements that take part in chemical reactions.