1. The Atom - Scientists

2. Philosophers Democritus – believed matter was not infinitely divisible THERE WERE NO EXPERIMENTS TO TEST HIS IDEAS Matter is composed of empty space through which atoms move Atoms are solid, indestructible and indivisible Different kinds of atoms have different sizes and shapes The different properties of matter are due to the size, shape and movement of atoms Changes in matter result from changes in the grouping of atoms and not from changes in the atoms themselves

3. Philosophers Aristotle – challenged Democritus’ ideas He believed the “nothingness” of empty space could not exist He denied the existence of atoms –He was so influential that his ideas were not challenged for 2000 years

4. John Dalton’s Atomic Theory (19 th Century) known as the MODERN ATOMIC THEORY HE WAS ABLE TO PERFORM EXPERIMENTS 1) All matter is composed of small particles called atoms 2) All atoms of an element are identical in size, mass, and chemical properties 3) Atoms cannot be created, divided into smaller particles or destroyed 4) Different atoms combine in simple whole number ratios to form compounds 5) In chemical reactions, atoms are separated, combined, or rearranged---What law? FALSE

5. 5 The Electron Streams of negatively charged particles were found to emanate from cathode- ray tubes (CRT). J. J. Thompson is credited with their discovery (1897).

6. 6 The Electron Thompson measured the charge/mass ratio of the electron to be 1.76 x 10 8 coulombs/g.

7. 7 Millikan Oil Drop Experiment Once the charge/mass ratio of the electron was known, determination of either the charge or the mass of an electron would yield the other.

8. 8 Millikan Oil Drop Experiment Robert Millikan (University of Chicago) 1909. –First determined the charge of an electron Electron – negative charged particle of matter (-1) –Next determined the mass of an electron Electron is so small it has a relative mass of 0 amu

9. 9 Radioactivity Three types of radiation were discovered by Ernest Rutherford:  particles  particles  rays

10. 10 The Atom, circa 1900 The prevailing theory was that of the “plum pudding” model, put forward by JJ Thomson. It featured a positive sphere of matter with negative electrons imbedded in it. INCORRECT MODEL

11. 11 Discovery of the Nucleus Ernest Rutherford shot  particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

12. 12 The Nuclear Atom Since some particles were deflected at large angles, Thompson’s model could not be correct.

13. 13 The Nuclear Atom Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. Later in 1920, he said the nucleus contained positively charged particles called protons. Most of the volume of the atom is empty space.

14. 14 Other Subatomic Particles James Chadwick – in 1932, he showed that the nucleus contained another subatomic particle that was neutral. The subatomic particle was called a neutron.