Key Dates in Medieval Europe


Democritus’ atomic theory posited that all matter is made up small indestructible units he called atoms.

400 BC

Democritus’s model stated that matter consists of invisible particles called atoms and a void (empty space). He stated that atoms are indestructible and unchangeable. Also that they are homogenous, meaning they have no internal structure. His atomic model was solid, and stated all atoms differ in size, shape, mass, position and arrangement, with a void exists between them.

Isaac Newton theorized a mechanical universe with small, solid masses in motion.


Newton formulated a theory of light, the three laws of motion and suggested a mechanical universe with small, solid masses in motion. Newton suggested that atoms are held together with attractions, otherwise known as forces. He also stated that "matter is formed of solid, massy impenetrable particles.

John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.


He proposed an atomic theory. Dalton stated (1) all matter was composed of small indivisible particles termed atoms, (2) atoms of a given element possess unique characteristics and weight, and (3) three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules).

Joseph John Thomson determined the charge to mass ratio of electrons.


The discovery of the electron and of isotopes, and the invention of the mass spectrometer. With the Nernst-Thomson rule he explains that it is difficult for charged ions to attract each other through insulating water molecules, so they dissociate.

Frederick Soddy came up with the term "isotope" to explain the unintentional breakdown of radioactive elements.


When Soddy came to Canada, he was given the opportunity to work with Ernest Rutherford, one of the leading chemists in radioactivity. For the next eighteen months, both Rutherford and Soddy discovered many things about the behavior of radioactive atoms. One of the most important behaviors they discovered was the transmutation of radioactive elements. Transmutation is the behavior of elements decaying into other elements while releasing energy.

Max Planck proposed the idea of quantization to explain how a hot, glowing object emitted light.


Planck, a German physicist, is considered the founder of the quantum theory.

Rutherford discovered alpha, beta, and gamma rays in radiation.


Ernest Rutherford was a chemist and a physicist who was also known as the father of nuclear physics
Contribution to our understanding of the atom: Rutherford discovered the concept of the radioactive half life. He also differentiated and named alpha and beta radiation. Rutherford is credited for performing the first splitting of an atom. Rutherford disproved Thomson’s theory of the atom in 1911, when he showed that atoms are mostly composed of empty space. If Thomson’s theory were correct, then the alpha rays should pass straight through the gold atoms. Instead, Rutherford found that some of the nuclei were deflected at large angles. A few were even deflected back to where they had come from.

Niels Bohr proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.


Fundamental contributions to the understanding of atomic structure and quantum mechanics. By expanding on Rutherford’s earlier theory Bohr published his theory which suggested that electrons travel in specific orbits around the nucleus, that outer orbits can hold more electrons than the inner orbits and that these orbits determine the chemical properties of the atom.

James Chadwick discovered neutrons, particles whose mass was close to that of a proton.


Famous for his discovery of the neutron. Chadwick discovered the neutron, named because of the lack of electric charge, which was previously an unknown particle in the nucleus of an atom.