1869, Dmitri Mendeleev
• Used cards to arrange the elements and created the first periodic table.
Late 1890s, Joseph John Thompson
• Discovered electrons using cathode ray test tube experiments
• “Plum pudding” model of atoms (1900)– negative electrons in a positive framework
1800, John Dalton, English chemist:
• Proposed the atomic patterns, which account for the law of conservation of mass and the law of constant composition
• All matter is made of atoms; atoms of an element are identical; each element has different atoms; atoms of different elements combine in constant ratios to form compounds; atoms are rearranged in reactions.
• “Billiard ball’ model – atoms are solid and indivisible
THE ATOMIC THEORY TIMELINE
500 B.C Greek philosophes
• everything belongs to a single unity
• Change & motion was an allusion
• Change & motion does exist,
• All nature existed as different object moving in a “void”.
1913, Niels Bohr
• Electrons orbit the nucleus in “shells” (electron configurations)
• Electrons can be bumped up to a higher shell if hit by an electron or a photon of light.
• Purposed the Bohr’s model & Bohr –Rutherford diagrams
1790, Joseph Priestly
• The first person who isolated elemental oxygen
• Isolated and named Oxygen and Hydrogen
• Discovered the law of conservation of mass (atoms are neither created/destroyed) & the law of constant proportions (elements combine in fixed ratios)
• Wrote the 1st chemistry book (1789) , listed 33 elements
• Grouped the elements
350 B.C, Aristotle
• Modified matter was made of the “4 elements” though the experiment “the burning of wood”.
1661, Robert Boyle, Alchemist
• The first person who defined an element