1. Chapter 4: Discovery of Atomic Structure

2. Prentice Hall © 2003Chapter 2 The Discovery of Atomic Structure An ancient Greek named Democritus was the first to say that matter consists of indivisible parts. Named parts of matter “atomos.” Later scientists realized that the atom consisted of charged particles.

3. Prentice Hall © 2003Chapter 2 –Each element is composed of indivisible atoms. –All atoms of an element are identical. –In chemical reactions, atoms are rearranged, not changed. –Compounds are formed when atoms of more than one element combine. –Which of these hold true today? John Dalton’s Atomic Theory of Matter

4. Cathode Ray Tube Experiment 1897: J.J. Thomson performs cathode ray tube experiment. Discovers electron. Plum pudding model of atom.

5. Prentice Hall © 2003Chapter 2 The Discovery of Atomic Structure Cathode Rays and Electrons

6. Thomson’s Plum Pudding Model The atom consists of neutral, positively, and negatively charged particles. Thomson assumed all these particles were found in a sphere.

7. The Discovery of Atomic Structure: Radioactivity 3 types of radiation: Alpha particles have 2+ charge Beta particles have 1- charge (electrons) Gamma rays have no particles, no charge

8. Prentice Hall © 2003Chapter 2 The Discovery of Atomic Structure Radioactivity

9. Gold Foil Experiment Ernest Rutherford carried out gold foil experiment. Alpha particles were shot through a piece of gold foil. Most of the  lpha particles went straight through the foil without deflection. Some  lpha particles were deflected at high angles.

11. Most alpha particles go through foil undeflected. Most of the atom must be empty space. Center of atom must be small, dense, and positively charged. Discovery of nucleus!

12. Rutherford changes Thomson ’ s Plum Pudding model as follows: –atom is spherical, has positive charge at center, electrons spread around center. Rutherford’s Nuclear Atom

13. Prentice Hall © 203 The Modern View of Atomic Structure The atom has protons, electrons, and neutrons. Protons have 1+ charge, are in nucleus. Electrons have 1- charge, are outside nucleus. Neutrons have no charge, are in nucleus. Protons and neutrons have almost same mass. Electron mass MUCH smaller. Nucleus is small and dense. Most of the mass of the atom is due to the nucleus.