The Birth of Atomic Theory

1. The Birth of Atomic Theory By: Mr. Hunter

2. Atoms • What is an atom? • How small do you think it is? • What is made up of atoms?

3. Atoms

4. Democritis • Called the smallest piece of matter an atom, which means “uncuttable”

5. Atoms • Element: a substance that cannot be broken down into simpler substances; first discovered in the 19th century • The smallest unit of carbon dioxide can be split apart into one atom of carbon and two atoms of oxygen. Is carbon dioxide an element? Explain.

6. By the end of the 19th century • 1. Scientists assumed that atoms were solid and indestructible, like tiny, hard balls. • 2. Scientists realized that each element was made of a different kind of atom. • 3. Scientists assumed that atoms contained no electrical charges.

7. Electrical Charges • When scientists exposed gases to electric fields, the motion of the atoms in the gases did not change. This seemed to confirm that atoms contained no charged particles. Why was this an incorrect idea?

8. Atoms • J.J. Thompson discovered a small, negatively charged particles and named them electrons.

9. Atoms • JJ Thompson proposed the plum pudding model

10. Atoms • Rutherford discovered that uranium emitted _____ and _____.

11. Apha Particles • Far more massive • Positively charged

12. Alpha Particles • Predict what will happen if an electron is brought near an alpha particle. • Predict what will happen if two alpha particles are brought near each other.

13. Rutherford • Rutherford aimed alpha particles at gold foil. • He hypothesized that all of the alpha particles would go through the thin foil and that most of the alpha particles would bend slightly as they interacted with the positively charged clouds of the gold atoms.

15. What were the results of Rutherford’s experiment? • 1. Alpha particles went through the gold foil. • 2. The paths of only a few of the alpha particles were bent by their passage through the foil. • 3. A few of the alpha particles bounced off the foil and came right back.

16. Rutherford’s results • How did his results contradict the plum pudding model of the atom? • Name one new finding that Rutherford’s model had to explain.

17. Rutherford • Rutheford’s experiment violated the plum pudding model. • How could atoms contain negative particles but still be neutral? • How could particles move through substances made of atoms?

18. Rutherford • Rutherford’s new model of the atom had to explain what could cause a fast-moving alpha particle to bounce back off a thin sheet of gold foil.

19. Modern Atomic Theory • What is the modern atomic theory? • Atoms are made of nuclei. Nuclei are made up of positively-charged protons, neutrally- charged neutrons, and surrounded by negatively-charged electrons.

20. Theories vs. Laws • Scientists use theories to make sense of a set of facts. • Any theory is open to questioning and testing. • Science is open to improving itself.

21. What is a law? • A scientific law is a description of a natural phenomenon or principle that invariably holds true under specific conditions and will occur under certain circumstances. • Ex.: Law of conservation of mass