Chapter 11- Introduction to Atoms

1. Chapter 11- Introduction to Atoms Section 1- Development of the Atomic Theory

2. Objectives Describe some of the experiments that led to the current atomic theory Compare the different models of the atom Explain how the atomic theory has changed as scientists have discovered new information about the atom

3. What is an atomic theory?? Atom- The smallest particle into which an element can be divided and still be the same substance Atoms make up elements, which make up compounds �Building Blocks of Matter� Theory- A unifying explanation for a broad range of hypotheses & observations that have been supported by testing

4. Is That a Fact?? It would take 1.05 x 1017 gold atoms to cover the entire surface of a dollar bill (105 quadrillion gold atoms) Along with contributing to the atomic theory, John Dalton was the first to describe colorblindness

5. Democritus Proposes the Atom Democritus (460-370 BC) felt that if you continually cut an object in half, soon you would end up with an �uncuttable� particle. He called this particle an atom (from the Greek atomos, meaning �indivisible�) Democritus� Principles All atoms are small hard particles of different shapes & sizes Atoms are always moving & form different materials by joining together

6. Aristotle Disagrees Aristotle (384-322 BC) disagreed with Democritus�s ideas Aristotle thought that you would never end up with an indivisible particle This idea was eventually been proven incorrect

7. Dalton Creates an Atomic Theory Based on Experiments By the late 1700�s, scientists learned that elements combine in specific proportions according to mass to form compounds John Dalton found that elements combine in specific proportions because they are individual atoms Dalton�s Theory All substances are made of atoms Atoms of the same element are exactly alike Atoms join with other atoms to make new substances

8. Was Dalton�s Theory Correct? Towards the end of the 19th Century Scientists agreed with many of Dalton�s observations However, as new information was discovered, Dalton�s theory was revised to more correctly describe the atom

9. Thomson Finds Electrons in the Atom In 1897, J.J. Thomson found an error in Dalton�s Theory Discovered that there are small particles inside the atom & therefore atoms can be divided into even smaller parts Cathode-Ray Experiment Positively charged plate attracts the beam The beam is made of particles with a negative charge

10. Thomson�s Cathode Ray Tube Experiment

11. Negative Corpuscles After repeating the experiment several times, Thomson found that the particles in the beam behaved the same each time, they look identical, & that they are present in every atom He called them corpuscles, today we call them electrons (e-)

12. Like Plums in a Pudding Thomsen revised the atomic theory to account for the presence of electrons He proposed a model to describe a possible structure. This became known as the �plum-pudding� model Today- Chocolate chip ice cream model Electrons are compared to the chocolate chips found throughout the ice cream

13. Rutherford Opens an Atomic �Shooting Gallery� In 1909, Ernest Rutherford designed an experiment to test the structure of an atom. He aimed a beam of small, positively charged particles at a thin sheet of gold foil To find out where the particles went after being �shot� at the foil, Rutherford surrounded the foil with a screen coated with zinc sulfide, which glowed when struck by the particles

14. Rutherford�s Gold Foil Experiment

15. Rutherford Gets Surprising Results Rutherford thought the particles would pass through the gold and continue in a straight line Most of the particles passed through Some were deflected a little Some were deflected a great deal Some seemed to bounce back

16. Rutherford Presents a New Atomic Model In 1911, Rutherford revised the atomic theory To explain deflection, he proposed that in the center of the atom is a tiny, dense, positively charged region (nucleus) where most of the atom�s mass is located Positively charged particles that passed close by were pushed away by the positive charge & those headed straight for it were turned back Rutherford calculated that the diameter of the nucleus was 400,000 times smaller than the diameter of a gold atom

17. Bohr States That Electrons Can Jump Between Levels In 1913, Neils Bohr suggested that electrons travel around the nucleus in definite paths. Paths are located @ certain distances from the nucleus (Figure 8, page 286) No paths between levels, but electrons can jump from one level to another Consider these levels like rungs on a ladder

18. The Modern Theory: Electron Clouds Surround the Nucleus Schrodinger & Heinsberg further explained the nature of electrons in the atom Electrons do not travel in definite paths as suggested by Bohr. The exact path cannot be predicted There are regions inside the atom where electrons are likely to be located (electron clouds)

19. Review Who came up with the �first� atomic theory? What does atomos mean? What is the name of the model Thomson proposed?

20. Assignment Answer the review questions on pages 283 and 286 Make a concept map comparing the different models of the atom