The smallest particle of an element is an atom. Each atom of an element has a certain number of protons, electrons and neutrons; once those numbers change, the atom makes a different element.
John Dalton was the first to propose that an atom is the smallest unit of measurement in physical elements. Afterwards, Ernest Rutherford used a series of experiments to prove that the nucleus of an atom in an element consists of a certain number of protons, neutrons and electrons. By understanding what each atom of each element is made of, chemists have a high degree of control over the chemical bonds they work to make. This allows humans to create new materials and new technologies.