The document traces the history and development of atomic models from ancient Greek philosophers to modern quantum theory. It describes Democritus's early idea of indivisible atoms, followed by John Dalton formalizing the first atomic theory in the early 1800s. J.J. Thomson later discovered the electron and proposed that atoms are like "plum puddings" with positive matter and embedded electrons. Ernest Rutherford's gold foil experiment revealed that atoms have a small, dense positive nucleus, leading Niels Bohr to model electrons orbiting the nucleus in fixed shells. Modern quantum theory describes electrons as occupying probability clouds or orbitals around the nucleus.

1. History of the Atom How did we figure out atoms exist? Who figured it out?

2. “ To understand the very large, we must understand the very small.” Greek Model Democritus’s model of atom No protons, electrons, or neutrons Solid and INDESTRUCTABLE No experiments to support idea Properties of atoms: indestructible but could change into different forms. an infinite number of kinds so there are an infinite number of elements. hard substances have rough, prickly atoms that stick together, liquids have round, smooth atoms that slide over one another. smell is caused by atoms interacting with the nose – rough atoms hurt. sleep is caused by atoms escaping the brain, death – too many escaped or didn’t return. DEMOCRITUS (400 BC) – First Atomic Hypothesis

3. DALTON’S ATOMIC THEORY I spent my lifetime trying to figure out how things combine, and now every sixth grader in the universe knows this stuff. John Dalton was the first to make the connection between atomism and recent discoveries in the lab. He used the theory that there were small, indivisible atoms to explain the law of definite proportions and the law of conservation of matter.

4. ALL ELEMENTS ARE COMPOSED OF ATOMS ALL ATOMS OF THE SAME ELEMENT HAVE THE SAME MASS, ATOMS OF DIFFERENT ELEMENTS HAVE DIFFERENT MASSES. COMPOUNDS CONTAIN ATOMS OF MORE THAN ONE ELEMENT IN A PARTICULAR COMPOUND, ATOMS OF DIFFERENT ELEMENTS ALWAYS COMBINE IN THE SAME WAY. IN CHEMICAL REACTIONS, ATOMS FORM ONE OR MORE COMPOUNDS OR ELEMENTS REDISTRIBUTE OR REARRANGE IN RELATION TO OTHER ATOMS TO FORM ONE OR MORE NEW COMPOUNDS. ATOMS THEMSELVES DO NOT UNDERGO A CHANGE OF IDENTITY IN CHEMICAL REACTIONS. DALTON’S ATOMIC THEORY This seems almost ridiculously simple now, but these scientists new nothing of atoms, protons, neutrons, etc… What Dalton did was to collect various works from many scientists and create this theory of matter. No easy task in a time without cell phones, cars, or even telegraphs.

5. Thomson Model of the Atom J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode ray tube. It is a vacuum tube - all the air has been pumped out.

6. Thomson’s Experiment + - Metal Disks Voltage source

7. Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment + - Voltage source

8. Thomson’s Experiment By adding an electric field + - Voltage source

9. Thomson’s Experiment By adding an electric field he found that the moving pieces were negative + - By adding an electric field Voltage source

10. J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these negative particles. He knew that atoms did not have a net negative charge and so there must be a positive charge balancing the negative charge. J.J. Thomson

11. Thomson’s Model Found the electron. Couldn’t find positive (for a while). Said the atom was like plum pudding. A bunch of positive stuff, with the electrons able to be removed.

12. Ernest Rutherford (1871-1937) Learned physics in J.J. Thomson’ lab. Noticed that ‘alpha’ particles were sometime deflected by something in the air. Gold-foil experiment Rutherford PAPER

13. Lead block Uranium Gold Foil Flourescent Screen Rutherford “shot” alpha particles (a type of radiation that is made of a helium nucleus of 2 protons and 2 neutrons) at a thin piece of gold foil.

14. Rutherford expected… The alpha particles to pass through without changing direction very much. Because… The positive charges were spread out evenly. Alone, a single positive charge was not enough to stop the bigger alpha particle with its two protons and much heavier mass. The alpha particle was like a bull in a china shop

15. So, Rutherfod expected…

16. Because

17. Because, he thought the mass was evenly distributed in the atom

18. What he got

19. Atom is mostly empty. Small dense, positive piece at center. Alpha particles are deflected by it if they get close enough. How he explained it +

20. +

21. Bohr Model After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbits around the nucleus. Planetary model Neils Bohr

22. Bohr’s Model Nucleus Electron Orbit Energy Levels

23. Modern View or Quantum Mechanical Model The atom is mostly empty space. Two regions. Nucleus - protons and neutrons. Electron cloud- region where you might find an electron.

24. Development of Atomic Models Rutherford model In the early twentieth century, Rutherford showed that most of an atom's mass is concentrated in a small, positively charged region called the nucleus. Bohr model After Rutherford's discovery, Bohr proposed that electrons travel in definite orbits around the nucleus. Thomson model In the nineteenth century, Thomson described the atom as a ball of positive charge containing a number of electrons. Quantum mechanical model Modern atomic theory described the electronic structure of the atom as the probability of finding electrons within certain regions of space.

25. Review Models of the Atom Dalton proposes the indivisible unit of an element is the atom. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. Thomson discovers electrons, believed to reside within a sphere of uniform positive charge (the “plum-pudding model). Rutherford demonstrates the existence of a positively charged nucleus that contains nearly all the mass of an atom. Bohr proposes fixed circular orbits around the nucleus for electrons. In the current model of the atom, electrons occupy regions of space (orbitals) around the nucleus determined by their energies.