Soddy worked with Ernest Rutherford to explain that radioactivity is due to the transmutation of elements. They showed that radioactive elements decay into other elements producing alpha, beta and gamma radiation.
He also showed that radioactive elements may have more than one atomic mass though the chemical properties are identical. He named this concept isotope, meaning "same place". Later, JJ Thomson showed that non-radioactive elements can also have multiple isotopes.
Credited with the discovery of the periodic table. Mendeleev organized the periodic table by atomic mass and saw that the physical and chemical properties of the elements repeated. He called this periodic law.
Democritus thought that matter could not be divided indefinitely. If you took a piece of matter and broke it in half, and in half again, and again, and again...then eventually you would no longer be able to break it apart. The smallest particle of matter he named atoms.