Development of the Atomic Theory

1. Development of the Atomic Theory SECTION 3.1 Pages 118- 130

2. ASSUMED KNOWLEDGE… • Subatomic particles • Structure and organization of periodic table • Trends (atomic radius, ionization energy and electron affinity)

3. THE GREEKS: DEMOCRITUS 400 BC • All matter is made up of small indestructible units he called atoms.

4. JOHN DALTON: 1809Atomic Theory • An atom is an indivisible sphere. • Atoms combine in specific mass ratios • Atoms have more than one combining capacity which results in more than one mass ratio Problems: Could not explain how atoms combined!!! Are atoms really indivisible? So… he was called “Hallucinating Crackpot”

5. DMITRI MENDELEEV: 1869 • Elements arranged on Periodic Table in order of increasing atomic mass • Similar elements grouped in columns or “families”

8. TODAY… • Elements are arranged in order of increasing atomic number, not mass • PT includes more elements that have been since discovered

10. J.J THOMSON: 1897 • Micheal Faraday (1832) and William Crookes (1879) had discovered that electrical charges were part of matter • Thomson used this info to explain the presence of a “negatively” charged particle with a mass of 1/1000 of an H atom (actually 1/1837) • Described the atom as being a positively charged sphere with electrons embedded. • No one knew about the proton yet! Raisin Bun Model

11. MARIE CURIE: 1898 • Worked with uranium and discovered that it emitted radiation (positive alpha, negative beta and gamma rays) • This radiation was in exact proportion to the amount of uranium. • Coined the term “Radioactivity” • Also discovered Radium and Polonium

12. Random Cool MARIE CURIE STUFF… • first woman in Europe to receive doctorate of science. • first woman to win a Nobel Prize for Physics. • first female lecturer, professor at the Sorbonne University in Paris (1906). • second Nobel Prize (chemistry) for discovery and isolation of pure radium and radium components. • She was the first person ever to receive two Nobel Prizes. • first mother-Nobel Prize Laureate of daughter-Nobel Prize Laureate. oldest daughter also won a Nobel Prize for Chemistry (1935). • first woman which has been laid to rest under the famous dome of the Pantheon in Paris for her own merits. • She received 15 gold medals, 19 degrees, and other honors.

13. ERNEST RUTHERFORD: 1911 • Studied radioactive elements also and won Nobel prize for this • Performed the gold foil experiment to prove Raisin bun model • Experiment: alpha particles aimed at gold foil; saw that a small number deflected back • Inconsistent with Thompson’s model • discovered that deflections were caused by encounters between alpha particles and electric field in centre of atom

14. GOLD FOIL EXPERIMENT

15. ERNEST RUTHERFORD: 1911 • Conclusions! • The atom is made up of mostly empty space • Small concentrated charged region in the centre that is positive (NUCLEUS) • Discovered the proton and realized that its mass was 1837x more than an electron

16. Positive nucleus with electron cloud • Nuclear model • Planetary model • FYI: • 1904 (Hantaro Nagaoka) also proposes “disc shaped” model and calls it “Saturnian model” Rutherford notes in journal that his results would not be the same if he had not read about Nagaoka’s model

17. ISSUES with Rutherford’s Model… • Total mass of atom could not be explained • Why don’t the positive particles in the nucleus repel and fly apart? • Why don’t electrons spin into the nucleus? • They should eventually fall into the nucleus and cause a “black hole effect”

18. BIGGEST PROBLEM with Rutherford’s Model… … • Laws of physics said that an electron in motion around a central body MUST give off radiation • Should be able to see a continuous spectrum (rainbow of light energy) as radiation is given off! • THIS was a HUGE turning point in the development of atomic structure

19. LISE MEITNER: 1945 • Discovered Nuclear fission • The nucleus can be split • Her Nobel Prize was given to Otto Hahn instead!!!

20. PLANCK 1900 • His idea was so crazy that he himself was unwilling to accept it! • Matter at the atomic level can absorb or emit only discrete quantities of energy • The energy of an atom (radiation) exists in chunks, or quanta! • Quanta are very small packets of energy and when they travel together they appear to be a wave

21. QUANTUM??? • Planck says that the energy of an atom is quantized • Something that is quantized can exist only in discreteamounts • Not continuous • Rungs of ladder vs hill of slope

22. EINSTEIN:1905 • Light energy is also quantized • Light occurs as a quanta of electromagnetic energy that have “particle like properties”; later called photons • Energy travels (as a wave) in the form of photons of energy

23. BOHR: 1913 Page 126: The Bohr Atomic Model 1) The atom has only specific, allowable energy levels, called stationary states. Each stationary state corresponds to the atom’s electrons occupying fixed, circular orbits around the nucleus 2) While in one stationary state, atoms do not emit energy 3) An atom changes stationary states by emitting or absorbing a specific amount of energy equal exactly to the difference in energy between the two states

24. WHAT IS BOHR saying? • The change in energy when an electron moves to higher or lower energy levels is NOT continuous • It is QUANTIZED! • Showed that the arrangement of electrons in energy levels around an atom could also be thought of in terms of quanta! • Electrons exist in fixed energy levels and are able to move from one level to another!

25. BOHR

26. Where is the neutron??? • Was not discovered until 1932 by Chadwick

27. READ ABOUT PROBLEMS WITH BOHR MODELPg 127-129

28. For timeline and More info on Scientists… • http://www.barcodesinc.com/articles/timeline-on-atomic-structure.htm