1 History – Democritus to Dalton Mr. ShieldsRegents Chemistry U01 L01

2 History of the Atom The earliest ideas in the development of atomic structure can be traced back over 2,000 years to the time of the Greek civilization And the whole story wasn’t completed until 1932 when the neutron was finally discovered!

3 source Democritus vs. Aristotle Two key players in the early history of the atom B.C460? – 370? B.C.

4 The Greeks Early Greek philosophers like Aristotle thought matter was made up of 4 different elements. Earth, wind, fire, water These 4 things having various Properties combined To form everything else in the world Matter could be divided forever Into smaller and smaller pieces And there was no such thing as Empty space.

5 Democritus Had a Different Idea Democritus – Proposed all matter is made up of indivisible particles He called them “Atomos”. He proposed … Atoms moved though empty space There are different kinds of atoms with different sizes & shapes The different properties of matter were a result of these differences Changes in matter were a result of changes in groupings of atoms not changes in the atoms themselves Not Bad for 400 B.C!

6 Democritus But … The idea of atoms were rejected by other Greek Philosophers like Aristotle “If everything was made up of particles with empty space between the particles with wouldn’t things like rocks or even people not fall apart?” The idea of the “ATOM” wasn’t re-investigated for almost 2 thousand years! During that time people went back to believing all things were made of Earth, Air, Fire and Water. Now let’s jump ahead 2000 years !

7 JOHN DALTON (1766 – 1844) 1803 –After studying the experimental observations of Lavoisier and others Dalton concluded matter could in fact be explained matter in terms of ATOMS A Revival of Democritus’ atomic theory Dalton’s Atomic Theory of Matter was based on the following 5 statements…

8 Dalton’s Atomic Theory of Matter 1. Each Element is composed of tiny indivisible particles called ATOMS 2. ALL atoms of a given element are identical; atoms of a specific element are different from other element 3. Atoms are neither created nor destroyed during Chemical reactions (law of conservation of mass as originally defined by Antoine Lavoisier in 1785) 4. In a chemical reaction atoms are separated, combined or rearranged And Lastly

9 Dalton’s Atomic Theory of Matter 5. Different atoms combine in simple whole number ratios to form compounds (as originally stated by Joseph Proust in 1799) This is called the LAW OF MULTIPLE PROPORTIONS EXAMPLE: H 2 + Cl 2  2HCL (1 atom of H always combines with one atom of Cl) If I find water in nature it’s H 2 0 and … it’s H 2 0 even when I make it in the Lab 2 H 2 + O 2 -> 2 H 2 O 2 atoms of H always combine with 1 atom of O to form 1 molecule of water

10 Dalton’s Atomic Theory of Matter Dalton’s Postulate’s formed the basis for our understanding of Chemistry today But Dalton and his contemporaries thought of Atom’s as “HARD AND ROUND” - in other words Atoms were like marbles But they had NO Concept WHY atoms of one element Behaved differently from the atoms of another element!