440 B.C.
Democritus and Leucippus proposed an idea of an atom

330 B.C.
Aristotle proposed that everything was a mixture of the four roots; earth, water, air, and fire
*Aristotle was Alexander the Great’s tutor

1865
John Newlands published the book, Law of Octaves which he arranged elements from their atomic mass
*Newlands can make an argument that he was the first to arrange the first periodic table

1869
Dmitri Mendeleev arranged the first widely accepted periodic table separating the elements by their properties

1897
J.J. Thompson performed experiments indicating that cathode rays really were unique particles, rather than waves, atoms or molecules as was believed earlier which he called electrons
*The charge and mass of electrons is E/M
!897
Ferdinand Braun crated the earliest version of a Cathode Ray Tube

1904
J.J. Thompson proposed the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges called Plum Pudding model

1905
Albert Einstein proposed the idea of a photon, in which the photon model accounted for the frequency dependence of light's energy
1909
Ernest Rutherford presented his model of an atom with a central charge surrounded by a cloud of electrons

1911
Ernest Rutherford discovered a nucleus in the atom due to Thompson’s previous efforts
*Rutherford has an element named after him, Rutherfordium

1913
Niels Bohr introduced his idea(Bohr Model)that the atom had a positively charged nucleus with electrons orbiting around it
*Named an element after Niels Bohr, Bohrium

1914
Henri Moseley found a relationship between an element's X-ray wavelength and its atomic number therefore